Which change is suitable for making red-orange warmer?

Correct answer: Add more red

Red-orange is a tertiary colour located between red and orange. Adding more red shifts its visual balance toward red, a warm colour, so option B is the most suitable change for making it warmer. Adding blue would introduce a cool influence, while grey would reduce saturation. Although yellow is also warm, extra yellow would shift the mixture toward yellow-orange rather than specifically strengthening its red warmth.