who is he ？a man___himself Smith

a man calling himself Smith.
= a man who calls himself Smith.
是现在分词作定语，等同于一个定语从句。“自称是史密斯的男士”
这句话也可变为：
a man called Smith
= a man who is called Smith
“一位叫Smith的男士”

回答是省略句，应该是He is a man calling himself Smith.已经有了谓语动词是be动词is，所以不能再有谓语，所以选calling

用calls的话要加who的，不然只能做一个陈述句来用。
a man calls himself Smith的意思是：那个人叫自己是Smith
而这个地方是要回答那个男人是谁
回答就说：那是一个称自己是Smith的人。诠释了这个人的信息。

其实整句话都写出来就是：a man calling himself Smith
也就等于: he is a man who calls himself smith

答案上的回答是一种简略说法