Answer: a security guard at the university.
The job is introduced as one of several possible occupations, so the indefinite article gives “a security guard.” The keyed sentence refers to a particular university, giving “at the university.” “At university” normally describes being a student, rather than identifying the institution where someone worked as a guard.
Article choice depends on context: “a security guard at a university” would also be grammatical in a different sentence. B is the official answer to this item.