Correct Answer: D
Explanation:
The first part of the clue is "mercurial character," which means George's moods change frequently. The second part of the clue is "one moment he was optimistic about his prospects," and the trigger is "the next he was." Thus, the blank should be the opposite of optimistic; look for words that mean pessimistic. Morose and dour are both similar to pessimistic. Hopeful and buoyant have the opposite meaning, and witty and immoral are not related.