What may happen if a very wet brush, rather than a damp brush, is applied to a wet watercolour layer?

Correct answer: The extra water may push the pigment and create an unwanted bloom

A very wet brush adds water to an already wet layer. If that extra water moves into the surrounding wash, it can push pigment aside and leave a bloom or backrun as the paint dries. Thus D is correct; a damp brush generally gives more control.