First, we can remove options A and D because they use "the security guard". This would only be right if he was the only guard or a very special one, which isn't true here.
Now, let's look at the remaining options:
B. "a security guard at the university"
C. "a security guard at university"
Both of these have a small problem. They say "the university" or just "university", but we don't know which university they're talking about. It could be any university in his city or even in another city.
The best answer would have been "a security guard at a university". But since we don't have this option, we need to choose between B and C.
Option C, "a security guard at university", is not very common in normal English. It's sometimes used in British English to mean "while attending university" rather than working there. So people might think he was a student who also worked as a guard, which is not given so we can't be certain about it.
Option B, "a security guard at the university", is better. It shows he was one of many guards at a specific university. Even though we don't know which university but this option is clearer and less confusing than C.
So, out of these choices, B is the best answer we can pick.
**from the information given by Claude AI as it is or edited.