TheUltimateWinnerIt's very normal to see things that CAN be verbs appear as modifiers. I'm sure you see this all the time! For instance, we might say "I stopped to talk with a professor considered by many to be the foremost authority on Mesopotamian pottery." How can we tell that "considered" is a modifier, and specifically an adjective modifying "professor"? First, it makes sense that the professor IS CONSIDERED an expert, rather than that the professor WAS CONSIDERING some other expert. The latter would be a strange and confusing thing to say. Second, a verb wouldn't work structurally here. Before "considered," we have a subject, a verb, and an object. We can't just tack on another verb. If we WERE writing about a professor WHO CONSIDERED someone else, we'd need to start a modifier before introducing a verb: "I met a professor WHO CONSIDERED Yoko Ono to be the greatest composer of all time."
Looking back at the original example, we can see that the same things are going on. First, nothing is ROOTING anything. Neither Monk nor the body of work ROOTED anything. In fact, no one can root a body of work in a tradition. To say that the work WAS ROOTED in a tradition just means that it HAS ITS ROOTS in that tradition. It's like saying a day was "filled with happiness." That doesn't mean someone actually filled the day with happiness. It's just describing the day. Second, if the work DID root something, we'd need a bridging modifier: "He created a body of work WHICH ROOTED." As it stands, all 5 choices say that the work was rooted, either by attaching ROOTED directly to WORK (A/C/E) or by saying "work that was rooted" (B/D). So in all the answers, ROOTED describes the body of work. That makes it an adjective.