Concept:The central idea or underlying message of a story or poem is its theme.
Explanation:The theme is the main idea or message that the writer wants to communicate.
This message may be about life, society, or human nature.
It is not the title, which only names the work.
It is not the climax, which is the highest point of action.
It is also not the conflict, which is the struggle between opposing forces.
The theme is expressed through characters, events, and language.
Therefore, the central idea of a story or poem is its theme.
Answer:C. theme