Answer:
If he's meeting his grandfather for the first time, then it's likely that he went to the past. So... It could be A (Don't take my full word for it) No matter what happens, he's going to be affecting the future. If he didn't, then it'd be a paradox, and that just gets even more confusing. Considering the grandfather is a "young man", I would say it's safe to assume that he won't be meeting himself while he's there. Answer (A) is inevitable here.
Explanation: