What may happen while filling colour if the paint contains too much water?

Correct answer: The colour may spread or run

Water changes the working consistency of paint. When there is too much water, the mixture becomes thin and less controllable, so it can spread beyond the intended area, form runs, or leave uneven washes; therefore option B is correct. Excess water does not make paint thicker, cannot harden paper as a normal filling effect, and cannot change the physical size of the brush. Controlled moisture gives cleaner coverage.