Answer: In this sentence, the word "optimistic" most clearly means "upbeat".
Explanation: The clue to the meaning of the word can be found in the contrast between the word "optimistic" and the word "dismal" that follows it. "Optimistic" means having a positive outlook or expecting a favorable outcome, while "dismal" means depressing or dreary. The sentence suggests that the girl is usually upbeat and positive, but her expression is currently the opposite.