The correct answer is C. Paco cumple seis años hoy
Let's say someone is talking to someone else they would say "cumple" not cumplen because that would mean that they are talking about more than one person. It wouldn't be "cumples" either because that would mean the person is talking directly to paco, and it just does not sound right. If it were to be "cumplo" it would have to have an "I" before the o because that would be past tense and paco is going to turn 6 he didn't already. Hope this helps