你好。答案的确是C.

在Four (multiplied) by seven is twenty-eight中， is 是系动词，twenty-eight是表语，four是主语，multiplied by seven 是後置定语，修饰four。你的句子裏面已经有了系表结构 is twenty-eight，所以A就不能选用了。

如果是A的话，句子是：Four, which is multiplied by seven, is 28. -------which is multiplied by seven是定语从句部分，被动语态结构。

如果是B的话，句子是：Four, which multiplies seven, is 28.-----which multiplies seven是定语从句，主动语态结构

D的话，是Four multiplying seven is twenty-eight.------multiplying seven是分词结构，主动概念。