A. amorous: This term refers to feelings of love or affection, which is not the opposite of "humorous." It's unrelated to the context of cartoons. B. grievous: This term means causing great pain or sorrow, and it is not the opposite of "humorous." It represents a serious and somber tone. C. hilarious: This term is similar in meaning to "humorous." It does not convey an opposite meaning; rather, it reinforces the idea of something being funny. D. grave: This term is the most appropriate opposite in meaning to "humorous." "Grave" refers to something serious, solemn, or dignified, contrasting with the light and amusing nature of "humorous." So, the correct answer is D. grave, as it provides the opposite meaning to "humorous" in the context of cartoons.