------ only by one’s appearance,I’m sure,will have you get j

选B,I am sure是插入语,通常此时可以先省去插入语看清句子结构,因此全句为Judging only by one's appearance will have you get just half of him or less.
如果是judge,judge only by one's appearance will have you get just half of him or less是不对的,因为直接动词开头不能做主语