In the second sentence of paragraph 18, the colon is used primarily to introduce a list of details after a general statement.
The sentence reads: "In the years when the frontier was being settled, the fence law came to the Middle West: farmers were required to fence their crops and livestock in and others’ out; otherwise, they were not entitled to legal protection of their property rights."
The colon is used to signal that the list of details that follows will explain or expand upon the general statement made in the first part of the sentence. In this case, the first part of the sentence provides the context of the fence law in the Middle West during the time of frontier settlement, and the second part of the sentence introduces the specific requirements of the law, namely that farmers were required to fence in their crops and livestock and keep others out in order to be legally protected in their property rights.
Therefore, the correct answer is A. introduce a list of details after a general statement.