For possession:
* Their's: This is an incorrect form of "their" and should be avoided.
* Theirs: This is the correct possessive pronoun for "they" and is a good substitute for "their" when referring to something owned by a group.
* His or her: This is a more formal way of saying "their" if you are referring to a group where the gender of individuals is unknown or mixed.
* Theirs/them: For example, "Their car is red" can be rewritten as "Theirs/them car is red". This is a less formal way of saying "their" and can be used in informal contexts.
For reference:
* Of them: This is a more formal way of saying "their" when referring to a group. For example, "Their thoughts on the matter" can be rewritten as "The thoughts of them on the matter."
* Those: This can be used as a synonym for "their" when referring to something that belongs to a group. For example, "Their house is big" can be rewritten as "Those house is big."
It's important to choose the synonym that best fits the tone and formality of your writing.