Answer:
[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 <em>their </em>does not agree in number with the antecedent <em>an enslaved person</em>. The revised versions are the following:
<em>It was common for </em><u><em>an enslaved person</em></u><em> to stop there on </em><u><em>his or her</em></u><em> way to the North.</em>
Or:
<em>It was common for </em><u><em>enslaved people</em></u><em> to stop there on </em><u><em>their </em></u><em>way to the North.</em>