I think the answer should be A. It is not mentioned which university the author is talking about.As the university in question is not referenced beforehand, the definite article "THE" cannot be used before "university". But "the" can be used before security guard,because we know that he was "the" security guard at some given university. Though ANSWER B sounds correct, I think the more appropriate answer is A. And there is no logical explanation as to why A should be wrong either .