Which sentence uses the correct form of 'there', 'their', or 'they're'?

The sentence "Their dog is very friendly" uses the correct form of 'there', 'their', or 'they're'. In this context, "their" is a possessive pronoun that indicates ownership, indicating that the dog belongs to a specific group of people. This usage effectively communicates that the dog is associated with the unspecified owners.

In contrast, the other options misapply the words: the first option incorrectly uses "there" instead of "they're" or a different expression, the third option uses "they’re" (which is a contraction for "they are") incorrectly in a context where possession is needed, and the fourth option also incorrectly utilizes "there" when it should use "their," indicating possession of the car. These distinctions are essential for proper grammatical structure and clarity in writing.