Which adjustment is least risky for warming a brown poster colour?

Correct answer: Add a tiny amount of red or orange

The governing concept is controlled colour mixing: brown can be made visually warmer by introducing a warm hue such as red or orange. A tiny addition is safest because it changes the hue gradually while preserving the existing value and saturation; the mixture can be checked after each small addition. A large amount of yellow may make the brown too bright or yellow, white mainly lightens it, and blue cools it. Therefore, option A is the most controlled correction.