Which record is most useful for reproducing an exact mixed poster colour later?

Correct answer: Colour ratio, water amount and a dry sample

The governing concept is reproducible colour mixing. A colour name is subjective, and a palette photograph may not show exact proportions or the final dried value. Recording the ratio of each pigment, the amount of water, and keeping a dry sample provides both process information and a physical reference. The brush size alone cannot identify the mixture, so option B is the complete record.