The correct option is C Subject: The cackling of geese; Predicate: saved Rome
The entire topic of discussion in the given sentence is the cackling of geese, not the cackling or the geese by themselves, not Rome, either. The sentence indicates that the cackling of geese did something, i.e., participated in action, which is what a subject does: does something. The predicate is whatever is being said about the subject, i.e., the work done by the subject. Therefore, the subject is the cackling of geese, and the predicate is saved Rome. Thus, option C is the correct answer.