如果不用从句的话此句可转换为 i sent invitation to 60 people.only 20 of them have replied.
在这里whom是代替of 后面的them.此句中间是逗号,明显,后面应为从句,如果硬要选B,则应该为：i sent invitation to 60 people.but only 20 of these peole have replied.
若选B,那么后面的句子only 20 of these（ who have replied）.就是定语从句,此句既是一个不完整的句子,只有主语20 of these,和定语who从句,没有谓语了.