Answer : repetitious
The question above is an example of an analogy. An analogy is one way of comparing things based on its structure. The first pair sullen and moody shows a synonymous relationship because the synonym of sullen is moody therefore, the next pair should be in a synonymous relationship also. It can be concluded that monotonous is to repetitious because the synonym of monotonous is repetitious. It can also mean dull, tedious or the one that lacks variety.