[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. [5] living in a piece of american history is pretty interesting.

- Sentence 2.

Explanation:

Pronoun antecedent agreement rules state that a pronoun must agree to its antecedent in number or else it leads to create confusion for the readers to understand it.

As per the question, sentence 2 contains a pronoun antecedent agreement error because the singular pronoun 'an enslaved person' does not agree with the plural antecedent 'their' in number. In order to rectify this pronoun antecedent agreement error, there are two ways that involve either the  conversion of the singular pronoun to plural or the plural antecedent to singular form as shown below:

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

or

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

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