The correct answer is C. Who is looked for by him?
The verb “look for” takes a direct object, which is the person or thing that is being looked for. In this case, the direct object is “him.” The preposition “by” is used to indicate the agent of the action, which is the person who is doing the looking. In this case, the agent is also “him.”
The other options are incorrect because they do not make sense grammatically. Option A, “Whom he is looked after for,” is incorrect because the preposition “for” is used to indicate the purpose of an action. In this case, there is no purpose indicated, so the preposition “for” is not necessary. Option B, “Who is looked after for him,” is incorrect because the preposition “for” is used to indicate the benefit of an action. In this case, there is no benefit indicated, so the preposition “for” is not necessary. Option D, “Whom he looked after by,” is incorrect because the preposition “by” is used to indicate the agent of an action. In this case, the agent is already indicated in the verb “look for,” so the preposition “by” is not necessary.