Let's break it down. The first part, Visual, is what you see when you look at something, color, arrangement, font, etc... The rhetoric part deals with the persuasion. In conclusion, it's what we see and how we act or think when we see it. It is one's ability to understand what an image is attempting to communicate.
Answer:
I do believe that the hardest part of an art critique is all based on opinion. Some have trouble with finding flaws, some with finding the good things. It all depends on you.
The correct answer is "COLOR"
Marvin Gaye would be the Prince of Motown :) I hope that helps.