What may be the main reason if some strokes appear bright and others dull while colouring a large area?

Correct answer: Uneven ratio of paint and water

The governing concept is consistent paint loading. If the paint-to-water ratio changes from one brush load to the next, some strokes deposit stronger pigment and look bright, while watery loads look lighter or duller. Option A is therefore correct. The handle colour, a written title and the brush’s round shape do not by themselves change the mixture strength. Preparing a uniform mixture helps keep a large area even.