Which mixture is most suitable for making a clean orange in poster colour?

Correct answer: Mix red and yellow on a clean palette

The governing colour-theory rule is that red and yellow are primary colours whose mixture produces orange, a secondary colour. Mixing them on a clean palette helps preserve a clear hue; residue from another colour can make the result muddy. Blue and yellow produce green, black and white produce a grey value, and red with green tends to reduce saturation rather than create a clean orange. Therefore, option A is correct.