An edge spread even though wet paint was applied on dry paper. What is a likely cause?

Correct answer: The brush contained excessive water

The governing concept is moisture control in a wet-on-dry watercolour application. Dry paper normally helps a painted edge stay relatively defined, but a brush loaded with excess water can deposit a watery pool that travels through the pigment and spreads beyond the intended boundary. A dark colour changes value, and a fine brush changes mark size, not the basic cause of spreading. Completely dry paper is not itself the cause, so option A is correct.