Are you familiar ----- the MP4 player they use?

C
be familiar with sth,对某物熟悉
be familiar to sb.对某人来说...不熟悉
C
后面的that is cheap and useful是一个定语从句,所以前面缺少一个名词或者代词作定语,我选one,指代前面的a birthday present.
I'd like to buy my mother a birthday present,one that is cheap and useful,意思是,我想买给我妈一件生日礼物,一件价格便宜又很有用的.