If the boundary of every brush stroke dries visibly in a wash, which error is indicated?

Correct answer: Waiting too long before applying the next stroke

The governing concept in an even wash is maintaining a continuous wet edge. If one stroke dries before the next overlaps it, the boundary remains visible and produces bands or patchy transitions. Thus option A identifies the error: the painter waited too long. Preparing enough paint may help, but it does not itself create a dried edge; brush size and paper angle are not the main cause.