Select the most appropriate idiom to fill the blank in the given situation. I thought Sadhna would always stick by me, but when I got into trouble, she turned out to be a/an ___________.

Correct — B. The sentence turns on but: expected loyalty in the first clause, desertion in the second, timed to "when I got into trouble".

A fair-weather friend is exactly someone who stays while things go well and disappears when they go badly. Fair weather stands for the good times, and the idiom says the friendship lasts no longer than the weather does.

Nothing else in the option set describes a person's loyalty at all, so option (b) is what the sentence requires.

Why the others are wrong

• (a)
  white elephant — A white elephant is a possession that costs more to keep than it is worth. It describes a burdensome thing, not a friend who walks away when you need her.
• (c)
  hard nut to crack — A hard nut to crack is a problem that resists solving, or a person hard to persuade. The speaker is complaining about desertion, not about difficulty.
• (d)
  open secret — An open secret is something widely known although it is supposed to be hidden. The blank has to describe Sadhna's conduct, and concealment is not what she is accused of.

Situation-based idiom items are decided by the pivot word in the sentence. Here it is but, which promises a reversal of whatever the first clause set up.

The first clause says stick by me, which is loyalty. So the blank must name the failure of loyalty, and specifically a failure that shows itself in bad times, because the sentence dates it to when I got into trouble.

Name what the blank must mean before you look at the options. Choosing among four idioms first is how this mark is usually lost.

Key facts

• Fair-weather friend: someone who is a friend only while circumstances are easy.
• White elephant: a possession that is useless or troublesome to maintain.
• Open secret: something widely known although it is supposed to be a secret.

Common traps

• Choosing hard nut to crack because Sadhna proved hard to read, when the idiom is about difficulty and not about disloyalty.
• Stretching white elephant to cover any unwanted thing, including a person.

No directly related past PYQ was found.