One cannot see through fog because

Light is scattered by the droplets in fog

Fog consists of tiny water droplets suspended in the air near the Earth's surface. When light encounters these droplets, it undergoes a phenomenon known as scattering. This means that the light is redirected in multiple directions, causing the fog to appear opaque or translucent. As a result, the light from objects doesn't reach our eyes directly, making it difficult to see through the fog. This scattering effect is commonly referred to as Tyndall scattering.

In contrast, the other options are not correct:

• a) Fog does not absorb light; it scatters it. Therefore, this option is incorrect.
• b) The refractive index of fog is not unity; it is slightly greater than 1. This option is incorrect.
• c) Light does not undergo total internal reflection at the droplets in fog; it scatters. This option is incorrect.

Understanding this scattering phenomenon helps explain why fog reduces visibility and makes it challenging to see through it.