I'm pretty sure it is Theme.
Theme is the central topic of the text. passage, story, or anything else. So it may possibly also be the same for the underlying meaning and universal truth...but again, I am not sure.
Answer:
tomb
Explanatio
Definition of word
A small room or monument carved into rock
C. irrelevant means of topic, doesn’t make sense with what’s going on, not useful
Answer:
The central idea is the main idea