In a fiction work, the theme is

Theme is the central idea or meaning of a story. Theme in fiction is rarely presented at all; it is abstracted from the details of character and action that compose the story. It provides a unifying point around which the plot, characters, setting, point of view, symbols, and other elements of a story are organized.

In works of fiction, a theme is the central idea or ideas explored in the story.

the answer is b. the tomb is underground and dark, and its opening is covered by a heavy grate.