To flog a dead horse means:

Correct option is A

The correct answer is option (a) – to do a vain thing.
Meaning of the given idiom: Flog a dead horse
• English: To waste effort on something that cannot be changed or has already failed
• Hindi: निष्फल प्रयास करना, व्यर्थ की कोशिश करना
Explanation:
This idiom implies continuing a discussion or effort that is pointless. Since a dead horse cannot be revived, trying to beat it would be useless – just as pursuing a failed matter is futile.
Other related idioms:
• Beat one’s head against a brick wall – व्यर्थ प्रयास करना
• Cry over spilt milk – बीती बात पर पछताना
• Bang one’s head against a wall – कुछ ऐसा करना जो कभी सफल न हो
Meanings of all the given options:
(a) To do a vain thing: To make a useless or fruitless effort.
• Hindi: व्यर्थ या निष्फल कार्य करना
• This is the correct meaning of the idiom.
(b) To be foolish: To lack sense or judgement.
• Hindi: मूर्ख होना
• Different meaning; not idiomatic match.
(c) To drag: To pull with effort.
• Hindi: घसीटना
• Literal; not idiomatic in this context.
(d) To revive interest: To regain attention or excitement.
• Hindi: रुचि फिर से जगाना
• Opposite of the idiom’s meaning.