If a stroke begins thick and ends watery, then which control is lacking?

Correct answer: Consistency of mixture and moisture in the brush

The governing concept is uniform brush loading. A stroke becomes thick at the beginning and watery later when the brush does not carry a consistent balance of pigment, medium and moisture. As the stronger mixture is used up, excess water reaches the surface and weakens the mark. Thus option D is correct. Paper size and hand direction are not the main causes, and colour alone does not describe the needed moisture balance.