The man who is now Municipal Commissioner worked as ____________________.

Answer: a security guard at the university

• Why this is correct: "a" introduces the job (a general role), and "the university" names the particular institution implied by the context. Together they form the natural phrase to describe a past occupation.

• "The security guard at a university" is awkward because it pairs a definite article for the person with an indefinite reference to the institution, which changes the intended general meaning.

• "A security guard at university" is not appropriate here because "at university" (without an article) typically means attending as a student, not working there.

• "The security guard at the university" could be correct only if the man had previously been identified as that single, known guard. Without such prior identification, the indefinite "a security guard at the university" is the natural choice.