Why do large and small salt grains create different effects in watercolour?

Correct answer: Grain size changes the area from which pigment is drawn

In the salt technique, grains are placed on a still-wet watercolour wash. Salt attracts moisture and pigment around the grain, so a large crystal influences a larger area and usually leaves a broader, stronger mark, while small grains create finer and more closely spaced marks. The final effect also depends on how wet the wash is and how much pigment it contains. Salt does not permanently fix the paint or create water. Thus option A is correct.