Final answer:
The correct possessive adjective in the sentence would be 'your' if the speaker is addressing the car's owner, and 'their' if referring to a person's car who is not being directly addressed.
Step-by-step explanation:
The sentence "hi, hetain is that your/their old car?" is asking about the ownership of an old car. The correct possessive adjective to use in this case depends on whether the speaker is talking to the owner of the car or about someone else's car.
If the speaker is talking directly to the owner, then "your" is the correct possessive adjective. If the speaker is talking about a third person's car, then "their" would be the appropriate choice. The possessive adjective 'your' indicates that the car belongs to the person being spoken to, while 'their' indicates that the car belongs to other people being referred to.
For example:
If referring to a car owned by Hetain but speaking to someone else: "Hi, is that their old car?"
Words that play the role of pronouns can sometimes function as adjectives, modifying nouns and pronouns. In 'That is my car,' the word 'that' is a pronoun, but in 'That car is mine,' it acts as an adjective describing the car.