What does the idiom 'To beat a dead horse' mean?

Correct option is D

Option (d) is the correct meaning of the given idiom.
Given Idiom: To beat a dead horse — to keep arguing or pushing a matter that has already been settled or is hopeless; to waste time and effort. (Hindi: व्यर्थ/निरर्थक विषय पर समय बर्बाद करना; मरे मुद्दे को उखाड़ना )
Example: Continuing this debate after the policy has been withdrawn is beating a dead horse.
Other related idioms and their meanings:
· Flog a dead horse — exact variant; to pursue a futile effort.
· Cry over spilt milk — lament what cannot be changed.
· Water under the bridge — past events that are no longer important.
· Barking up the wrong tree — pursuing a mistaken line of action.