Why should the poster colour container be closed after use?

Correct answer: To prevent the paint from drying

The governing concept is proper storage of a water-based painting material. Closing the container after use limits exposure to air, so moisture evaporates more slowly and the poster colour remains usable for longer. Thus, option D is correct. A closed container does not change the container’s colour, shorten a brush, or dry paper. The lid should be replaced firmly after taking out paint, and the rim should be kept clean so it seals properly and prevents premature drying.