Step 1: Understand the idioms and meanings.
- (A) “The balloon goes up” means the situation becomes serious, chaotic, or critical. Spot fixing scandal is a serious, chaotic event. Hence, this pair matches.
- (B) “In the altogether” means being naked. Bath tub is a place where people are naked. Correct association.
- (C) “The halcyon days” refers to a tranquil, happy time in the past. Childhood is often seen as a happy period. Correct association.
- (D) “The real McCoy” means something genuine or authentic. Darjeeling Tea is known for authenticity. Correct association.
- (E) “Throw down the gauntlet” means to issue a challenge or an invitation to fight or competition. UN peace keeping force is about maintaining peace, not about issuing challenges. This does not fit.
Step 2: Identify the odd one.
Options (A), (B), (C), and (D) are proper idiomatic pairings. Option (E) is mismatched because the meaning of the idiom does not correspond to the given phrase.
\[
\boxed{\text{Answer: E (Throw down the gauntlet : UN peace keeping force)}}
\]