Answer :
The sentence that has the correct sentence structure is C: Apartment hunting is one of the most frustrating activities that a college student undertakes. This is so because it follows the typical sentence structure: S(subject)-V(verb)-predicate (in this case).
The main verb of the sentence is the verb "to be" for the third person singular (is). This verb links the grammatical subject of the sentence with an adjective or noun that relate to the subject. Therefore, in this case, the verb to be takes an NP subject (apartment hunting) and a predicative nominative, an NP that tells something about the subject (one of the most frustrating activities that a college student undertakes).
Sentece is wrongly punctuated, sentence B has a marked structure and sentence D is not grammatically correct.