D. That all aspects of the piece should work together to create a certain effect
To create a "unity of effect" on the reader, it should have a certain effect on the reader. Like a really sad and depressing book should not exactly make the reader sad, but to feel the parts of the book that spoke out to the reader.
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.
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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. </span>