can I help you?I would like to buy a present for my father`s

选C.
I would like to buy a present for my father`s birthday,--------at a proper price but of great use.
后半句 “at a proper price but of great use” 没有动词,不是一个完整的句子,所以不选连接词which 和 it,而one可以做代词,指代 present.the one 的话有特指的意思,这里还没有选好礼物,不用特指.