which their is correct

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https://www.merriam-webster.com/words-at-play/how-to-use-theyre-there-their

They’re, There, and Their: Correct Usage | Merriam-Webster
https://www.merriam-webster.com/words-at-play/how-to-use-theyre-there-their
Their, there, and they’re are all pronounced the same way. Their is the possessive pronoun, as in their car is red; there is used as an adjective, he is always there for me, a noun, get away from there, and, chiefly, an adverb, stop right there; they’re is a contraction of they are, as in they’re getting married.

https://www.wikihow.com/Use-There,-Their-and-They’re

How to Use There, Their and They’re: 8 Steps (with Pictures)
https://www.wikihow.com/Use-There,-Their-and-They’re
Use their and theirs to indicate possession. Compare to heir — to inherit possessions as in an heir of their uncle who had no children. Their is a possessive adjective and indicates that a particular noun belongs to them. Sam and Jan are heirs of their parents’ land and possessions as their legal heirs.

https://www.grammarly.com/blog/there-their-theyre/

How Should I Use There, Their, and They’re? | Grammarly
https://www.grammarly.com/blog/there-their-theyre/
Their is the third-person plural possessive pronoun: All of those kids with their contagious laughter really made my day. They didn’t want to see all of their hard work go to waste.

https://www.dictionary.com/e/their-vs-there-vs-theyre/

Their vs. There vs. They’re: Do You Know The …
https://www.dictionary.com/e/their-vs-there-vs-theyre/
There has the word here in it. There is the choice when talking about places, whether figurative or literal. They’re has an apostrophe, which means it’s the product of two words: they are. If you can substitute they are into your sentence and retain the meaning, then they’re is the correct homophone to use.

https://www.grammar.com/their-correct-spelling

their – correct spelling – grammar
https://www.grammar.com/their-correct-spelling
This Grammar.com article is about their – correct spelling — enjoy your reading! Note: For a discussion of their, there, and they’re, see Grammar.com’s section on Common Grammatical Mistakes. Click here for the beginning of that discussion. Example: They’re not taking their best clothes over there.

https://www.merriam-webster.com/dictionary/their

Their | Definition of Their by Merriam-Webster
https://www.merriam-webster.com/dictionary/their
They’re (=they are) two of our biggest problems. The last of this trio, their, is the possessive form of they, so it has to do with what belongs to, relates to, or is made or done by certain people, animals, or things: It’s their house. We’re their neighbors. The trees are losing their leaves. And there you go.

https://grammar.yourdictionary.com/grammar-rules-and-tips/rules-for-there-and-their.html

Rules for Using There, Their and They’re
https://grammar.yourdictionary.com/grammar-rules-and-tips/rules-for-there-and-their.html
The English language is full of commonly confused words that trip up native speakers and new learners alike. Many sound the same, but have different spellings and meanings, such as there, their and they’re.. Luckily, the rules for the difference between there and their, as well as the contraction they’re, aren’t difficult to remember.

https://www.grammar-quizzes.com/there-their.html

There vs. Their vs. They’re | Grammar Quizzes
https://www.grammar-quizzes.com/there-their.html
THEIR They (a subject pronoun) and their (a possessive pronoun) are personal pronouns that take the place of a personal noun. The neighbors are bringing their children too. (possessive.) They are bringing their children too. (possessive.)

https://www.sirene.fr/sirene/public/accueil?sirene_locale=en

There, They’re, Their – The Easiest Way To Learn How To …
www.theretheyretheir.com
Use their to show possession, commonly followed by a noun. Usually, if you can replace their with our in the same sentence, and it still makes sense, then you are using it correctly. Examples Their dog is pretty awesome. Have you seen their new store?

https://www.grammarbook.com/grammar/pronoun.asp

Pronouns | Types of Pronouns and Rules
https://www.grammarbook.com/grammar/pronoun.asp
The use of they and their with singular pronouns is frowned upon by many traditionalists. To be consistent, it is a good practice to try to avoid they and its variants (e.g., them, their, themselves) with previously singular nouns or pronouns. Not consistent: Someone has to do it, and they have to do it well.