What is a key difference between a theme and a central idea?

a. a central idea is specific to a genre, such as epic poetry, while a theme is specific to a medium, such as written text.
b. a central idea is relatively neutral, while a theme makes some kind of moral point.
c. a central idea is a concept that a written work suggests, while a theme is a concept that it states directly.
d. a theme is shown by characterization or plot, while a central idea is shown by rhetoric or stated directly.

the answer is d. the theme is the central message in the text. the plot and the characterization build up the theme. thus it is not direclty mentioned in the text.themes are universal can be taken out and applied to other texts.

whereas the   central idea is stated directly in the text. the reader learns what   the story is about to the central idea.main ideas are not universal because they only apply to the text they belong. we can say they are unique.

in ray bradbury's short story "a sound of thunder", mr. eckels's fearfulness leads to drastic consequences for the rest of the world.

in the story, mr. eckels, who is a hunter, hires a time travel company to travel to the age of dinosaurs. although he is excited for the hunting expedition at the beginning, when they reach there and mr. eckels sees the tyrannosaurus, he feels frightened. as he is afraid of the tyrannosaurus, he steps off the path although he has been warned not to do it. by stepping off the path, the entire course of the future is affected: mr. eckart alters the future by stepping on a butterfly, which causes a new man to be elected president in the future. in that way, it is mr. eckart's fearfulness what triggers 'the butterfly effect' and what leads to drastic consequences for the rest of the world.