un·prec·e·dent·ed<span>ˌənˈpresədən(t)əd/</span>adjectivenever done or known before."the government took the unprecedented step of releasing confidential correspondence"<span>synonyms:<span>unheard of, unknown, new, novel, groundbreaking, revolutionary, pioneering, epoch-making; unparalleled, unequaled, unmatched, unrivaled, without parallel, without equal, out of the ordinary, unusual, exceptional, singular, unique; formalunexampled"warfare on an unprecedented scale"</span></span>
Explanation: This is the only answer that explains exactly what the essay is about. A and D only talk about one branch, not all three. If you use A or D as a topic sentence the reader will think the essay is only about one branch. B talks about the different names of the federal government , not the branches themselves which may lead the reader to think that the essay is about the different names of the federal government.
