What is the possessive form of "they" used before a noun?

The correct answer is "their" because it functions as the possessive pronoun for the plural pronoun "they." When indicating that something belongs to a group of people, "their" is used in front of a noun. For example, in the phrase "their car," the word "their" shows that the car belongs to them.

The other options do not serve this grammatical function. "There" is an adverb indicating a place, while "they're" is a contraction of "they are," used to describe a state of being. "Thy" is an archaic form of "your," used in older English texts and does not apply to modern usage for the pronoun "they." Understanding these distinctions clarifies why "their" is the appropriate choice for expressing possession related to "they."