Which colour must be present in a painter's basic colour box?

Correct answer: Red

The relevant concept is the role of primary colours as the basic set for traditional painting. Red is one of the three primary colours, along with yellow and blue, so it is an essential choice for a basic colour box and option A is correct. Grey, brown and pink can be useful in particular artworks, but they are not members of the traditional primary-colour set and can often be prepared or adjusted from other colours.