An ______________ found that one of the marathon runners had ______________ slowed down in the last two kilometres and after ______________ to the one behind him had let him cross the finish line just one second before he did.

The passage describes a sportsmanship incident in a marathon where one runner intentionally allowed another to finish ahead. From the context, it is clear that a noun meaning "formal examination" is needed for the first blank; an adverb indicating intentional action for the second blank; and a verb meaning "communicating non-verbally" for the third blank.

Among the provided options, "investigation" (Option D) is the only word that precisely fits the first blank, as it specifically refers to an official inquiry into the race's events—the appropriate term for examining competitive conduct. "Analysis" (Option A), although fitting, is too generic; "appraisal" (Option B) suggests evaluation rather than fact-finding, and "overview" (Option C) implies a summary rather than close examination.

For the second blank, "deliberately" (Option D) stands as the optimal choice, as it means "intentionally", which is the exact adverb needed here to describe the runner's conscious decision. "Systematically" (Option A) suggests methodical action rather than sportsmanship, "particularly" (Option B) doesn't indicate intent, and "purposely" (Option C), while close, is less formal than what the context demands.

The third blank requires "signalling" (Option D), which specifically means "communicating through gestures" - the precise mechanism for how one runner yielded to another. "Waving" (Option A) is too casual for a competitive context, "speaking" (Option B) contradicts the nonverbal requirement, and "beckoning" (Option C) implies summoning rather than yielding.

Hence, option D is the correct choice here