We are talking about Tom,_____you met in the street yesterday.填什么关系词

引导定语从句的词有关系代词that ,which ,who (whom宾格 ,whose所有格)和关系副词where ,when ,why.
这里先行词是Tom,人,所以必定是用人称代词that(主格)或 who（主格）/whom（宾格）
因为 定语从句的意思是 you met him in the street yesterday,所以him是宾格,只能用whom来引导.所以正确句子如下：
We are talking about Tom,__whom___you met in the street yesterday.