very few people understood his speech,---was complicated.A.t

是选A.做这种定语从句的题目,你放到原文中去还原就行了.
the subject of his speech（which指代的就是his speech）was complicated,符合句意.
which subject 的话放到文中.speech subject was complicated不对,speech跟subject 应该是从属关系,用whose subject 才对.