I think you should combine c and d if possible, if not then a is the best answer in my opinion.
Answer:
A theme is the <em><u>message </u></em>conveyed in the text.
Explanation:
Any written work of literature will have an underlying message that it wants to relay to the readers. This allows the central message to be the focus around which the whole story develops.
The theme of any literary work or speech or any conversation is the subject or message that the whole passage/ text revolves around. This enables the overall understanding of what the author or writer intends to share with the readers. Thus, in short, we can say that the theme of a text is the message that it intends to convey to the readers.
