What is a likely cause if dark edges are forming between brush strokes?

Correct answer: The next stroke was applied after the first one dried

A dark edge is commonly caused by uneven drying and pigment collecting at the boundary of a stroke. If a new wet stroke is placed beside a partly dry or dry stroke, it re-wets the edge and creates a visible darker line, so option D is correct. Brush size, paper whiteness, and handle length do not normally create this drying boundary effect.