Choose the odd one:

Checking option one by one.

Option A:  The balloon goes up means the situation becomes serious, critical, chaotic, or troublesome. Spot fixing scandal are serious, critical, chaotic, or troublesome. 

Option B: In the altogether means without any clothes on; naked. Bath tub is used when someone is naked. 

Option C: The halcyon days means a tranquil period of happiness, especially in past.  Childhood is a period of time when everyone was usually happier than present. 

Option D: The real McCoy means Something  is genuine, authentic, or exactly what it is claimed to be; real thing. Darjeeling Tea is a sign of authenticity. 

Option E: Throw down the gauntlet means To issue a challenge or invitation, as to a fight, argument, or competition. Here the authors talks about UN peace keeping force which is not related to throw down the gauntlet. 

Hence, option E is the correct answer.