Is this the reason _____ at the meeting for his carelessness in his work?

问题描述:

Is this the reason _____ at the meeting for his carelessness in his work?
A.he explained B.what he explained C.how he explained D.why he explained

A.he explained
这是定语从句,先行词是reason,关系代词在从句中做宾语,已省去.