[1] our house was once a stop on the underground railroad. [2] it was common for an enslaved person to stop there on their way to the north. [3] the underground railroad were a network of hiding places. [4] the cellar beneath our house has a secret door that was installed to trick slave catchers.

which sentence has a pronoun-antecedent agreement error?
sentence 1
sentence 2
sentence 3
sentence 4

[2] It was common for an enslaved person to stop there on their way to the North.

Explanation:

Pronoun-antecedent agreement refers to the correspondence in number between the pronoun (which stands for a noun) and the antecedent (which is the word the pronoun stands for). As a result, a singular pronoun must agree with a singular antecedent, and a plural pronoun must agree with a plural antecedent.

In the example sentence, the pronoun their does not agree in number with the antecedent an enslaved person. The revised versions are the following:  

It was common for an enslaved person to stop there on his or her way to the North.

Or:

It was common for enslaved people to stop there on their way to the North.

the answer is d: message a character's thoughts