The past participle is a form of a verb that usually ends in "-ed" (when they're regulars) and that can be used as adjectives, to form the passive voice (For example, she was carried by her father) or to form perfect verb tenses such as the future perfect, the present perfect and the past perfect, the latter being formed with the auxiliary verb <em>have</em> in past: <em>had</em> + the past participle of the main verb.
So, since the sentence provided is not in passive voice (It is not focusing on the recipient of the main action), and the blank is not missing an adjective but a verb, the blank should contain the past participle of "argue" that forms a perfect verb tense, more specifically, the past perfect: "had argued."
A thesis statement is the main idea of an essay. It consists of the topic of the essay and the writer's claim about the topic that will be proven throughout the essay.
