For singular individuals:
* His/Her: If you know the gender of the individual, use the appropriate pronoun.
* His or Her: If you don't know the gender, use this to avoid assuming.
* Theirs: This is grammatically correct but can be awkward if it's clear there is only one individual.
* The person's: This is more formal and can be useful if you're unsure about the gender.
For groups or plural nouns:
* Their: This is the correct pronoun for groups of people, animals, or things.
* Theirs: This can be used in place of "their" when referring to something belonging to them.
* The group's: This can be used to emphasize the group as a single entity.
Other options:
* Those: This can sometimes substitute "their" when referring to a group or a specific set of things.
* One's: This is more formal and can be used in place of "their" when referring to something belonging to someone in general.
Example:
* Original: Their dog is very friendly.
* Substitute: His/Her dog is very friendly (if you know the dog owner's gender).
* Substitute: The dog is very friendly (if you don't know the dog owner's gender).
* Substitute: Their dog is very friendly (if you are referring to a group owning the dog).
Ultimately, the best substitute for "their" will depend on the context and the message you're trying to convey. Choose the option that is most accurate, clear, and avoids unnecessary assumptions.