How can watercolours be prevented from becoming muddy?

Correct answer: By using a clean brush, clean water and a clean palette

Watercolour remains clear when pigments are kept clean and are not unintentionally contaminated. A clean brush, fresh water, and a clean palette prevent residues of one colour from entering another, so the washes retain their intended hue and brightness. Repeatedly mixing many colours, adding black indiscriminately, or using an unwashed brush introduces unwanted pigment and commonly produces dull, muddy mixtures. Hence option A is correct.