B
enough 修饰形容词或副词时是：形容词/副词+ enough
修饰名词时是 ：enough+ 名词
fast和slowly是副词,排除A和D
fast是快地,slowly是慢的,根据句子意思：别担心.我能跑的足够_____ 来赶上我的同学们.
能够赶上说明应该是跑的够快,所以选fast enough