The question asks for the meaning of the English idiom: 'To flog a dead horse'.

Idiom Meaning Explained

The idiom 'To flog a dead horse' refers to continuing to put effort into something that has already failed or cannot be changed. It signifies pointless exertion.

Analyzing the Options

• Option 1 (to whip a dead horse) is the literal interpretation, not the idiomatic meaning.
• Option 2 (attempt to do the possible) contradicts the idiom's sense of futility.
• Option 3 (waste one's efforts) accurately captures the meaning of putting energy into something that yields no results because it's impossible to succeed or change.
• Option 4 (to take advantage of a weakness) is unrelated to the idiom.

Conclusion

Therefore, the best expression of the idiom's meaning is that it involves wasting one's efforts on a hopeless or unchangeable situation.