Why should brushes be washed quickly while working with acrylic colour?

Correct answer: Paint may dry on the brush and harden the bristles

Acrylic paint remains water-mixable when wet, but it dries quickly into a firm film. If it is left on the brush, the paint can bind the bristles together, make them stiff, and permanently reduce the brush’s usefulness. Rinsing the brush during work and immediately afterward prevents this buildup. Washing does not make a brush heavy, keep paper wet, or brighten the paint, so option B is correct.