‘To flog a dead horse’ means:

• We infer that flogging a dead steed is futile and ineffective
• Hence it's also a waste of energy and energy on a commodity that is formerly gone.
• Therefore, to grasp the precise meaning of the expression given to us within the question, we must be suitable to match the meaning of it with what we have inferred from the hint.
• The commodity that is futile can't be made less or more practical. So, option A is the incorrect option.
•  To be accepted satisfactorily does not fit the environment of our conclusion. Hence, option B is the wrong answer.
• To waste one’s powers on a drive matches our conclusion of the expression. 
• Then, the cause represents the dead steed and flogging the dead steed may be a waste of energy. Therefore, we're suitable to say that option C is conceivably an accurate answer.
• To exercise, another person does not fit the environment of our conclusion. Hence, this is an incorrect option.
•  After observing all the choices and thus the possible meanings, we conclude that option C is the correct answer.

Hence, option C is the correct answer.