Answer:
A). The poems theme.
Explanation:
A theme is usually described as the central idea, message, or lesson of the author that he/she intends to convey through the work. Such a message or idea is underlying and needs to be inferred.
As per the question, the reader can learn about the theme of the poem after studying the implicit meanings of words in a poem as a poem employs figurative language that carries a connotative meaning which could be understood only in the context provided by the author. Thus, option A and i.e. 'the poem's theme' is the answer.
