The correct answer is (C).
Getting to the Answer:The non-underlined key word "and" that immediately precedes the underlined portion creates a compound. The parts of a compound must be parallel, so both of the connected phrases must be in identical grammatical form. Since the phrase before the "and" is in the form "his legs tumbling ...," the second phrase must include "his torso" and an -ing word.
(A) Incorrect: "whose torso crashed" is not in the same grammatical form as "his legs tumbling"
(B) Incorrect: "who?s torso crashed" is not in the same grammatical form as "his legs tumbling"
(C) Correct: "his torso crashing" is in the same grammatical form as "his legs tumbling"
(D) Incorrect: "its torso crashing" is incorrect because the boxer is human, and the pronoun "it" cannot be used to refer to a human being