Answer:
B). It is the underlying message that a poem conveys.
Step-by-step explanation:
The theme is the central idea of the author that he wants to convey to the audience which could be an underlying message, thought, or lesson which is being explored throughout a literary work. It exemplifies the central claim of the author that drives the plot of a poem or any other literary work to propose a universal idea or message. The theme is the essential element of a literary work that offers meaning to it and at the same time, it binds the several elements of the work together to be presented as a whole. Thus, the best definition of a theme within poetry would be the underlying message that is being conveyed through the poem by the poet.