Which is the simplest exercise for learning the primary colours?

Correct answer: Filling red, yellow, and blue

A direct colour-identification exercise is the simplest way to learn the primary colours. Filling separate shapes with red, yellow, and blue gives the learner repeated visual and practical contact with the three names and their appearances. Therefore, option D is correct. Sharpening a pencil, tearing paper, and kneading clay may support other art skills, but they do not directly teach the primary-colour set.