The correct option is B b
The relative pronoun functions as a pronoun as well as a conjunction. It refers or relates to the noun that's common between two sentences and combines those sentences. In the given sentence, "the old man" is the subject, "helped" is the verb and "me" is the object, the subject performs the action against the object. Here, the subject is the antecedent, or the noun that the relative pronoun relates to the rest of the sentence, as the speaker mentions the old man and the rest of the sentence is in relation to him. When the antecedent is the subject, the relative pronoun 'who' is used, whereas the object antecedent uses 'whom'. Since the (b) part contains 'whom' and not 'who', it has the error, and we choose Option B that has the error. The (a) part contains a demonstrative that refers to "the old man", and has no error, hence we don't choose A. The (c) part is correct, it refers to a past action where the old man helped the speaker "me", it has no error, hence we don't choose option C. We don't choose option D as well, as the sentence has an eror in (b).