While creating rock texture with dry brush some areas become solid opaque patches. What is the main cause?

Correct answer: The brush contains more paint or moisture than required

Dry-brush texture depends on a nearly dry brush carrying only a small amount of paint, so the raised surface catches broken marks while recesses remain partly open. Excess pigment or moisture fills those gaps and produces solid opaque patches, making C correct. Light pressure normally preserves broken texture; paper texture and a light colour tone are not the main fault.