I can't understand ____ at that poor child.

I can't understand you laughing at that poor child.我不理解你嘲笑那个可怜的孩子.
这道题是考察动名词的复合结构
动名词的复合结构有四种形式：①形容词性物主代词＋动名词；②名词's＋动名词；③代词宾格＋动名词；④名词＋动名词.
动名词的复合结构可在句中作主语或宾语.作主语时,不能用③、④两种形式；无生命名词不能用第②种形式,无论是作主语还是作宾语.动名词的复合结构,实际上是给动名词加了一个逻辑主语.如：
His／Jack's not getting to the station on time made all of us worried．他／杰克没准时到火车站使得我们大家都很担心.
Do you mind my／me／Jack's ／Jack leaving now 我／杰克现在离开你介意吗?
mind .doing 我们见得到.此处understand you laughing 也是动名词的复合结构,理解你嘲笑.

固定用法啊
can not understand sb's doing sth

I can't understand you laughing at that poor child
=I can't understand you (who are) laughing at that poor child 分词做后置定语
=I can't understand you (who) laugh at that poor child分词做后置定语的另一种形式