Ironically, before he shot and killed Abraham Lincoln at Ford’s Theatr

This Tense/Timeline problem highlights the most important verb tense of all on GMAT, the past perfect ("had shot"). Note that "had performed" is already in the non-underlined portion, situating the performance of John Wilkins Booth further back in time before he shot the president. This means that A correctly conveys the timeline by putting "shot and killed" in the simple past tense - still back in the past but after past perfect. B is incorrect because it doesn't provide a past-tense event, which is required in order for the past-perfect to exist in the later sentence. C is guilty of the same error, as is E. And choice D is redundant - "prior to" doesn't need the word "when" after it, because the idea of time is already inherent in "prior to". So D is incorrect, as well, and choice A is the proper answer. (Sidenote - this isn't really "ironic", but in a nod to Alanis Morrissette we'll let the question stand)