正确答案是B.
1.原句采用了定语从句来修饰the biggest trouble.如果把定语从句写完整,则是：
I came across the biggest trouble _____ English.
2.此处come across=meet,意思是“遇见,碰到”.
此结构类似于以下固定短语：
sb have trouble/difficulty with sth.
=sb have trouble/difficulty (in) doing sth.(in可以省略)
某人在（做）某事上有困难.
3.所以此处应填入：
I came across the biggest trouble（in) studying English.
所以答案是B.