选B.
这是就“强调句中被强调的部分”进行提问后的句子.
假设那个人是Mary.则原来的句子为：
Mr.Smith saw Mary in the shopping mall the other day.
根据变成强调句的规则：“It is/was + 被强调部分 + that +句子剩余部分.”
则就Mary进行强调后的句子为：
It was Mary that Mr.Smith saw in the shopping mall the other day.
然后就Mary进行提问,就得到：
Whom was it that Mr.Smith saw in the shopping mall the other day?
所以选B.