Using the principles of Propositional Logic (Truth-tellers and Liars puzzle), the correct answer is Option 1 [Ela].
Step-by-Step Logical Proof:
1.
Conditions:
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Exactly 3 people always speak truth (T), and exactly 2 lie (F).
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Exactly 1 person is the guilty culprit.
2.
Identify the Contradiction:
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Ali: "Dev committed the offence."
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Dev: "Ali is lying when he accuses me."
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Ali and Dev make mutually contradictory statements. Exactly one of them is Truthful (T) and the other is a Liar (F).
3.
Analyze the Remaining Suspects (Cynthia, Ela, Banu):
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Since exactly 1 truth-teller comes from {Ali, Dev}, the remaining 2 truth-tellers must come from {Cynthia, Ela, Banu}, while 1 is a liar.
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Ela states: *"Banu is telling the truth."*
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If Banu is T, Ela is T.
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If Banu is F, Ela is F.
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Therefore, Ela and Banu share the identical truth value. Since we need exactly two truth-tellers in this group of three, both Ela and Banu must be Truth-tellers (T), and Cynthia must be the Liar (F)!
4.
Identify the Culprit:
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Cynthia's statement is: *"Ela is innocent."*
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Since Cynthia is a proven liar (F), her statement is FALSE.
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Therefore, Ela is NOT innocent $\implies$ Ela is the Culprit!
Verification:
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Culprit = Ela.
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Banu ("It wasn't me") $
ightarrow$ True.
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Ela ("Banu is telling the truth") $
ightarrow$ True.
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Ali ("Dev did it") $
ightarrow$ False.
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Dev ("Ali is lying") $
ightarrow$ True.
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Cynthia ("Ela is innocent") $
ightarrow$ False.
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Truth-tellers = 3 (Banu, Ela, Dev); Liars = 2 (Ali, Cynthia). The system is fully consistent!
Identify direct contradiction pairs first (Ali vs Dev). This immediately isolates the truth-value distribution for the remaining statements.