The answer to your question is A
Which of the following solution(s) would be a buffer? Select all that apply.
a. 100.0 mL of a solution that contains 0.100 M benzoic acid and 0.100 M potassium benzoate
b. 100.0 mL of a 0.100 M propanoic acid
Third person plural pronoun is THEY.
If you want to create a possessive pronoun out of that one, that would be THEIRS.
Possessive pronouns, as the name itself says, determine some kind of possession. For example:
Is this books yours? - No, it's theirs.
The woman takes her jacket off and hangs it up in her closet
