Answer:
English settlers
Step-by-step explanation:
The antecedent of a pronoun is the word to which the pronoun refers, which comes before it. As the antecedent is the word that the pronoun refers back to, the two have to agree in number for the sentence to make sense. In this case, the pronoun "they" refers to the people that built forts and towns. These were the "English settlers." The pronoun and the antecedent agree as they are both plural.