D, sentence 4. "Next, a huge, cumbersome crank..." is how the sentence should be written. "huge" and "cumbersome" are two adjectives in a row, which is one hint that it's your answer; however, in sentence 2, you also see "a lot like giant wooden boxes"--and "giant" and "wooden" are both adjectives. sentence 2 doesn't require a comma because wooden applies to boxes, as in the specific type of box, and "giant" modifies the "wooden boxes" as a whole. the same cannot be said for the "huge, cumbersome crank" because "huge" doesn't modify the "cumbersome crank" as a whole, it only modifies the ceank itself.
