Explanation
Correct answer: D.
Choice D is the best answer. The conventions being tested are punctuation use between sentences and the punctuation of a supplementary element. This choice correctly uses a period to mark the boundary between one sentence ("During...one") and another ("The Katzenbach...event") and uses a comma to separate the supplementary phrase "for one" from the preceding main clause. Further, placing the period after "for one" correctly indicates that the information in the preceding main clause ("the founding...1929") is the first example provided of a pivotal event in the Latino rights movement.
Why the other choices are wrong
Choice A
Choice A is incorrect because placing the period after "1929" illogically indicates that the information in the next main clause (describing the Katzenbach v. Morgan court decision) is the first example provided of a pivotal event in the Latino rights movement; rather, it’s a second example.
Choice B
Choice B is incorrect because it results in a comma splice. A comma can’t be used in this way to join two main clauses.
Choice C
Choice C is incorrect because it results in a comma splice. A comma can’t be used in this way to join two main clauses. Moreover, it fails to use appropriate punctuation to separate the supplementary element "for one" from the preceding main clause.