What should be checked first if a watercolour mixture becomes muddy?

Correct answer: Cleanliness of the brush, water, and palette

A muddy watercolour mixture often results from contamination: a brush may retain a complementary or unrelated pigment, the rinse water may be dirty, or the palette may contain residue from an earlier colour. These unwanted pigments mix and reduce clarity. Paper length, the subject, and the name of the colour do not explain sudden muddiness. The first practical check is therefore the cleanliness of the brush, water, and palette, followed by using fresh, controlled mixtures. B is correct.