Minutia

A. a significant idea

B. an influential person

C. a simple explanation

D. a set of instruction

Minutia means a small piece of information which might not be entirely significant. Its plural form is "minutae".

Sample sentence: It’s essential that everyone should know the overview of the entire system; however, you need not to focus on the minutiae of everything.

Basically, minutia is a small detail, so (A) a significant idea and (B) an influential person are obviously not the answers. Between C and D, C can be the closest because of the word "simple". A minutia can be an explanation or instruction but its core essence if insignificant. So, C is the best answer.