If a dry tertiary-colour sample appears darker than the wet sample, how should the next mixture be adjusted?

Correct answer: By making a slightly lighter mixture

Many wet paint mixtures appear lighter or more reflective than they look after drying. If the dried test sample is darker than the intended result, the next batch should be adjusted slightly lighter, then tested again on the same surface. Thus A is correct. Adding black or an opposite colour would usually darken or dull the mixture, while repeating it without checking ignores the observed result.