I couldnt him of his mistakes.

A

I couldn't convince him of his mistakes.

A.

情态动词后加动词原型

A convince
couldn't 是情态动词，所以后面必须跟动词原形。另外
convince sb of sth 固定搭配

选A convince sb of是个短语

A，couldnt是情态动词，情态动词后面应该接动词原型。