Question

He is the only person--i believe,can help you with your Engl

Answer 1

B,这句话要很小心,He is the only person [who (i believe) can help you with your English].
我用括号一标,可以发现这句话的主干是：He is the only person,定语是[who can help you with your English].（I believe）为插入语,可以省略.
再问： 假如没有who，那改选那个which可以吗
再答： 不行。如果把定语部分改成一句话：He can help you with your English.相当于对“he”部分划线提问，你只能用“who”提问， who can help you with your English，不能用“which”提问，所以不能用which。
再问： 题目上给了A.whomB.whatC.thatD.which该选哪个
再答： 那就只能选C了...因为the only要加that...