Here's why:
* Demonstrative adjectives point out specific nouns. They are used to show which one or ones you are referring to. The common demonstrative adjectives are: *this, that, these, those*.
* "This" in the sentence "Which one is this?" is acting as a demonstrative adjective, pointing out a specific hat.
Let's break down the other options:
* "His hat" - "His" is a possessive pronoun, not a demonstrative adjective.
* "Whose this?" - This is grammatically incorrect. "Whose" is a possessive pronoun, and "this" needs to be followed by a noun.
Therefore, the only sentence with a demonstrative adjective is "Which one is this?"