The central idea, concept, or subject of a literary piece is referred to as the piece’s

The central idea, concept, or subject of a literary piece is referred to as the piece’s:

Theme

Explanation:

The literary "theme" is the phenomenon or study object that will be the base of every text. This will define the language, thee arguments and the posture to speak on the text. Thanks to the "theme" the writer will know enough information after researching it and will also be able to express an opinion about it. The texts use arguments to defend the thesis which is the posture of the writer about the "theme" using knowledge and learnings obtained after the writer's studies.

a.) literature that explores grotesque characters and eerie, violent occurrences in a dreary setting.