But my friend ---swin faster than me A doB does C can选哪个？为什么？

和上下文都有关
B 和 D 都可以啦，大概要选 can：

I am a good swimmer, but not as fast as my friend - （but） my friend can swim faster than me

You can win this race,you are the best! Oh, I am a good swimmer but my friend does swim （游泳么？） faster than me. (expressing doubt as to the final outcome)
does 用来强调它比你好

明白吗？

b does 你朋友是第三人称
加does是为了加强语气,就像I do love you 意思还一样但更强烈

But my friends can swim faster than me

同意二楼观点
如果强调我朋友“会”游泳，用can
如果强调我朋友“游泳比我快”，用does
英文不能断章取义，结合上下文理解

A吧,DO做助动词,表强调.
但我朋友确实比我游得快.