Which correction is suitable if a watercolour skin tone becomes too dull?

Correct answer: Add a little clean red or yellow

A dull or lifeless skin tone usually needs a small amount of a clean warm colour to restore freshness. Red or yellow can add warmth and liveliness when used carefully. More grey, black, or a larger quantity of a complementary colour generally reduces saturation further and may produce a muddy mixture. The adjustment should be gradual because watercolour becomes lighter as it dries. Therefore, option D is correct.