Hi, Ahmed,
Hi, "My father.......me a present on my birthday party."
a- will buy b-is buying
Both are grammatically correct. Using 'will' means that it is just your expectation or your prediction. Using 'is buying' means that it is arranged. You can also use 'is going to buy'. BTW, we have a similar question in a conversation in our books.
Ziad: This is my cousin’s e-book reader, but I really like it, so my father is buying me one for my next birthday.