When a pronoun refers to a noun, the noun and pronoun should agree in number. If the noun is singular, the pronoun should be singular. If the noun is plural, the pronoun should be plural.
Compare the following pairs of sentences:
My cousin leads an exciting life because she visits exotic places.
Travel writers lead exciting lives because they visit exotic places.
My cousin, who is a travel writer, leads an exciting life because they visit exotic places.
In the first example, the singular noun cousin and the singular pronoun she agree. In the second example, the plural noun writers and the plural pronoun they agree. In the third example, the singular noun cousin and the plural pronoun they do not agree.
The noun that a pronoun refers to is called the pronouns antecedent.
In which sentence does the pronoun correctly agree with its antecedent?