University

Sentence a) is correct if you happen to be in England or perhaps India.
There they say someone is in hospital, or in university, Americans add a "a", thus sentence b) is correct in the US.

So it will depend on your reader rather than any particular acceptability.

Joe

Joe's right about the British use: "at university" is a state of existence/education, like "at school".

"at a university" would be used with emphasis on the word "university", to contrast is with some other institution (technical college, for example).

A widely used abbrevation, which I don't like much myself is "Uni" for "university". e.g. "I'm going back to uni in September".

KP

Additionally, colloquial American uses the word "college," even though that may be most often used as a term for a subdivision of a university. The usage would be:

"I haven't seen Bill since he left High School, where is he now?"

"Oh, he went off to college, he's a brainy type, ya know."

Many Americans will simply use the word college, even when referring to a university.

Hmm, Set, I've heard some Chinese student who went to Australia and studied there said that Australian college is actually the same as Hight School in China, while a university is the same level as China's.
So in Australia, should we not call a uni student as a college student?

I spoke only of an American colloquial usage, Boss . . .