The correct answer is B. In poetry, the theme is often suggested with symbols.
If you are not sure, use elimination. A is incorrect because sometimes characters can reveal the theme (when these options have leading words such as 'never' or 'always,' they are usually incorrect). C is incorrect because usually the theme is implied. D is incorrect because characters are often emotional in novels, just like humans truly are.
Therefore the correct answer is B, as symbols can often tell us a lot about the theme.