What should be done to keep clear separation between colours?

Correct answer: Apply the next colour after one area dries

The relevant acrylic-painting principle is controlled layering. Allowing one painted area to dry before applying an adjacent colour prevents wet pigments from spreading into each other and helps preserve a sharp boundary. Mixing all colours would remove separation, pouring water would disturb the paint, and rubbing the paper could damage its surface. Thus, option B gives the correct practical method.