Which mixture should be avoided to keep red poster colour bright?

Correct answer: Too much green in red

The governing concept is complementary colour interaction. Red and green are complementary in the usual colour-wheel system; mixing a large amount of green into red reduces chromatic intensity and can produce a dull, muddy brownish result. Therefore, option B should be avoided. A little white lightens red, a little yellow can warm it, and pure red preserves its strongest brightness.