What is the most likely reason if a poster-colour mixture loses brightness after drying?

Correct answer: Mixing too many colours and making it muddy

The governing concept is saturation loss caused by overmixing pigments. When many poster colours are combined, several pigments absorb and scatter light in competing ways, often producing a dull, muddy mixture with reduced brightness. Thus option C is correct. A clean brush helps prevent contamination, white paper generally supports brightness, and mixing paint less does not by itself explain a muddy dried result. A limited palette usually gives cleaner colour.