Which option best explains the meaning of the English idiom "to flog a dead horse" as it is used in everyday and exam contexts?

Introduction / Context:
Idioms are fixed expressions whose meanings cannot always be understood by literally translating the individual words. The idiom "to flog a dead horse" appears often in newspapers, conversations, and competitive exams. Understanding its figurative meaning is essential for strong vocabulary and good performance in English sections of competitive tests.

Given Data / Assumptions:
- The idiom is "to flog a dead horse".
- We must select the option that best captures its established figurative meaning.
- The options suggest ideas about foolish action, wasted effort, reviving interest, and criticism.

Concept / Approach:
Literally, "flog" means to beat or whip, and "a dead horse" is an animal that can no longer move or work. Beating a dead horse will not make it stand or work again. This image symbolises continuing to argue about, work on, or push something that has no chance of success or has already been settled. Therefore, the idiom means "to waste one's efforts" on a hopeless or finished matter.

Step-by-Step Solution:
Step 1: Visualise the literal picture: a person beating a horse that is already dead.
Step 2: Understand that no amount of beating will bring the horse back to life or make it move, so the person's effort is completely wasted.
Step 3: Connect this image to typical usage, such as "We are flogging a dead horse by discussing this issue again; the decision has already been made."
Step 4: Among the options, "To waste one's efforts" matches this meaning most closely and consistently.

Verification / Alternative check:
Consider how the idiom is used in news reports or office conversations: "Arguing with him about this is just flogging a dead horse" means that the person will not change their mind and further discussion is pointless. It does not mean reviving an old subject effectively, and it is not mainly about criticism or memories. The essence is hopeless, useless effort applied to something that cannot be changed.

Why Other Options Are Wrong:
- To act in a foolish way: while the behaviour is foolish, the idiom specifically emphasises wasted effort on a hopeless task, not general foolishness.

- To revive interest in an old subject: this is almost the opposite, because flogging a dead horse usually fails to revive anything.
- To revive old memories: relates to remembering the past, which is not the idiom's meaning.
- To criticise someone harshly: flogging can metaphorically mean harsh treatment, but the fixed idiom has a more specific meaning about useless effort.

Common Pitfalls:
Some learners misinterpret the idiom as "bringing an old issue back" because they focus on the idea of an old horse. However, the key point is that the horse is dead, so the action cannot achieve any result. Always remember that this idiom is used when further effort is clearly pointless and unproductive.

Final Answer:
"To flog a dead horse" means to waste one's efforts on something that cannot succeed or change.