Question

That is a boy... which parts of speech is that​

Answer 1

Pronoun.

Explanation:

From the definition of pronoun we get, pronouns are those words which are used instead of nouns or names or any word which sits instead of a noun or name or identifies a person instead of his/her name is termed as pronoun.

"That is a boy", here we are not using the name of the boy.

Here 'that' is indicating the  boy and instead of his name we are using 'that'. So, 'that' is a pronoun here. Here, the word, 'that' is able to make a complete sense of the sentence or the clause.