Author dara strolovitch states in the unit essay that the dominant two-party system of the us is an unintended outcome of

The correct response is Option D. First-past-the-post election rules and single-member districts.

Explanation:

The first-past-the-post-idea makes it so the losing party has no representation at all in Congress, so it promotes a winner takes all outcome that has helped to propagate the system in the United States that is dominated by only the two strongest parties. Authors like Dara Strolovitch argue that single-member districts like that in the United States tend to promote two-party systems. This  called Duverger's law where to have a multi-party system you need proportional representation.  The cube rule is another concept that fleshes out this idea: the party who wins in a first-past-the-post system is over represented and all the people who did not vote for the winning candidate are not represented at all.  

the answer would be in this answer. someone already asked the question.