What is the the demonstrative pronoun in this sentence Which of these is the car that you were hoping to buy?

The demonstrative pronoun is these, a Word that takes the place of a plural noun or two or more nouns.

The pronoun 'which' is an interrogative pronoun, a Word that introduces a question.

The pronoun 'that' is a relative pronoun, a Word that introduces the relative clause 'you were hoping to buy'.

The pronoun 'you' is a personal pronoun, a Word that takes the place of a noun for the person spoken to.