Here's why:
* Demonstrative adjectives point out specific nouns. They include words like "this," "that," "these," and "those."
* A. One is this: "This" is used to point out a specific "one."
* B. This his: "This" is used incorrectly here. It should be "This is his."
* C. hat: This doesn't contain a demonstrative adjective.
* D. Whose this?: This is a question, and "whose" is a possessive adjective, not a demonstrative adjective.
Therefore, only sentence A contains a demonstrative adjective.