Choose the meaning of the given idiom- A fair weather friend
(a) A friend who is fair to us at all times
(b) A friend who deserts us in difficulties
(c) A friend whom we love the most
(d) A friend who loves us the most

 Hint: An idiom is an expression whose meanings cannot be inferred from the meanings of the words that make it up.
For example- Takes two to tango.

Complete answer :
In the given question, the phrase ‘a fair-weather friend’ refers to (idiomatic) one who is friendly, helpful, or available only when it is advantageous or convenient to be so.

Let us analyze the options given to us in this question:
1) Option (a), 'a friend who is fair to us at all times', refers to a friend who’s free from favoritism or self-interest or bias or deception.
For example- He never favours me over Sheila. He is a friend who is fair to us at all times.
2) Option (b), ‘a friend who deserts us in difficulties', refers to a friend who tends to leave someone who needs or counts on him/her; a friend with self-interests.
For example- When I needed someone to step up and lend a helping hand, I expected Amy would do so. Unfortunately, she is truly a friend who deserts us in difficulties.
3) Option (c), ‘a friend whom we love the most', refers to a friend who’s an object of our warm affection or devotion.
For example- Nina is the most helpful and charming person in the group. She is a friend whom we love the most.
4) Option (d), ‘a friend who loves us the most', is completely irrelevant to the meaning of the idiom ‘a fair-weather friend’.
For example- Michael is a very devoted person towards this friendship. It would not be wrong to say that he is a friend who loves us the most.

Hence Option (b) is the correct answer.

Note: Notice that ‘a fair-weather friend’ has the noun ‘weather’. It can refer to changing mood or actions or convenience in this context, and not the atmospheric conditions.
Thus, always remember that you may get a clue about the idiom’s meaning from the words that make it up, but never the whole meaning!