Red light has the longest wavelength among all colours of visible light. Due to its long wavelength, red light is least scattered by fog, smoke, or dust particles in the atmosphere. Therefore, red light can travel the longest distance without significant loss of intensity and is visible clearly from far away. This makes red colour ideal for danger signals at airports and tall buildings, ensuring they are visible even in poor weather conditions.
Source: Chapter 10, Section 10.6 – Scattering of Light
The key concept here is scattering of light — shorter wavelengths (violet, blue) scatter more, longer wavelengths (red) scatter least. Examiners expect you to explicitly state: (1) red has the longest wavelength, (2) it is least scattered by fog/smoke, and (3) therefore visible from the greatest distance. Don't just say "red is not scattered" — say least scattered and link it to visibility from a distance. The textbook itself uses this example directly in Section 10.6.