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Description

Lockepicke looks above the doorway behind her and sees a tiny chest. Her eyes glance up at Face on the Wall. He is still looking amicably down at her.
 
“Hey, maybe your master ended up in this chest!” Lockepicke says.
 
“But he isn’t even big enough to fit in that…” Face on the wall says.
 
The mare has already opened the chest at this point however, and revealed a small stash of coins. She’s a bit reluctant to take the coins now, but makes sure to remember them for later.
 
As Lockepicke passes the placard, Face on the Wall says, “Sorry about the guys up ahead. They are kind of jerks.”
 
The stairs eventually end at a door with a very large lock on it. The doorway is flanked on both sides by orbs on pedestals marked from A to D. Each pedestal has a small cabinet set into it.
 
“Oh-ho! One for the test then, is it?” The orb marked A says flashing, “Well it’s simple enough. One of us contains the key for this door. The others contain~~”
 
“I swear if you say ‘explode’ I’ll smash you all on the ground.” Lockepicke says, glaring.
 
“Hmm… Well, the others contain material that will detonate. How’s that, then?” The orb marked C says flashing, “Anyway, let’s get this test started.”
 
—~~
 
One of the cabinets contains the key for the next room! Here’s what the Orbs have to say.
 
A: “B has the key!”  
B: “D is hiding the key!”  
C: “I don’t have the key! Not me!”  
D: “B is lying!”
 
Only one of these Orbs is telling the truth. Which one contains the key?
 
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Background Pony Number 17
My Little Pony - 1992 Edition

Whether one of the orbs is telling the truth or not obviously depends on which one is holding the key. So we have to examine each possible situation.
 
If A is holding the key, then  
A is lying, B is lying, C is telling the truth, D is telling the truth.  
2 truth-tellers, 2 liars.

 
If B is holding the key, then  
A is telling the truth, B is lying, C is telling the truth, D is telling the truth.  
3 truth-tellers, 1 liar.

 
If C is holding the key, then  
A is lying, B is lying, C is lying, D is telling the truth.  
1 truth-teller, 3 liars.

 
If D is holding the key, then  
A is lying, B is telling the truth, C is telling the truth, D is lying.  
2 truth-tellers, 2 liars.

 
The only scenario where only one of the orbs is telling the truth is the one where C is holding the key.
mathprofbrony
Nightmare in the Moon - Had their OC in the 2024 Derpibooru Collab.
Solar Supporter - Fought against the New Lunar Republic rebellion on the side of the Solar Deity (April Fools 2023).
Roseluck - Had their OC in the 2023 Derpibooru Collab.
Elements of Harmony - Had an OC in the 2022 Community Collab
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Friendship, Art, and Magic (2020) - Took part in the 2020 Community Collab
Wallet After Summer Sale -
Friendship, Art, and Magic (2018) - Celebrated Derpibooru's six year anniversary with friends.
Birthday Cake - Celebrated MLP's 7th birthday
The End wasn't The End - Found a new home after the great exodus of 2012

@flashman07
 
We can tell from the condition that only one orb is telling the truth.
 
Any other orb having the key would mean multiple orbs were telling the truth. The only way A is telling the truth is if B has the key, hence B’s statement is false, hence D’s statement is true, but both A and D cannot be telling the truth.
 
Actually, even without A’s statement, the key in a different orb would mean two orbs were telling the truth anyway.
flashman07

Queen Chrysalis Minion
@Crowley  
I think so…that is the solution I came up with as well…D is telling the truth and A, B, and C are lying, which means that C was lying about NOT having the key (so C has it).
 
The only thing that bothered me a little was that none of the lies or truths point back to A.
 
We still don’t know if A is telling the truth or lying…because nothing points back to it.
Crowley
Wallet After Summer Sale -
Magical Inkwell - Wrote MLP fanfiction consisting of at least around 1.5k words, and has a verified link to the platform of their choice
Birthday Cake - Celebrated MLP's 7th birthday
The End wasn't The End - Found a new home after the great exodus of 2012

Can't we all get along?
Okay, I think I figured this out. Bare with me.
 
||  
It’s all a process of elimination. If only one of the Orbs are telling the truth, then we have to go with the scenario within which only one of those statements is true. So…
 
If A is telling the truth…  
A = True  
B = False  
C = True  
D = True
 
If A was being truthful, C’s and D’s remarks would also ring true. And since we know only one can be true, we can safely assume A’s a liar. Let’s try it again with the other scenarios.
 
If B is telling the truth…  
A = False  
B = True  
C = True  
D = False
 
Again, if B’s saying D has the key, then C would also be telling the truth in that it doesn’t have the key. Two truthers in the puzzle mean B’s a liar.
 
If C is telling the truth…  
A = False  
B = False  
C = True  
D = True
 
D saying B’s lying would also be true… therefore again, two truthers mean the scenario isn’t C. Which means…
 
If D is telling the truth…  
A = False  
B = False  
C = False  
D = True
 
This is the only scenario where assuming the True Orb doesn’t contradict the others. Therefore we know that D is certainly the truth-teller.
 
And as an added bonus, we now know all the others are definitely lying. Which means C is lying when it says it doesn’t have the key.
 
My final verdict: D is telling the truth out of them all, and C has the key.  
||
 
Did I get it right?