Which of the following contains small droplets of liquid or particles of solid dispersed in a gas?

The correct answer is Aerosol.

Colloids are mixtures in which microscopic particles of one substance are dispersed evenly throughout another substance. Colloids are classified into different types based on the physical state of the dispersed phase (the substance that is dispersed) and the dispersion medium (the medium in which it is dispersed).

Let us analyze the definitions of the given options:

1. Aerosol: An aerosol is a colloid in which small liquid droplets or solid particles are dispersed in a gas medium.
Examples include fog, mist, spray (liquid dispersed in gas), and smoke, dust (solid dispersed in gas).

2. Gel: A gel is a colloid where a liquid is dispersed in a solid medium.
Examples include jelly, cheese, and butter.

3. Foam: A foam is a colloid where a gas is dispersed in a liquid (or solid) medium.
Examples include whipped cream, shaving cream, and soap suds.

4. Vapour: Vapour refers to a substance in the gas phase at a temperature lower than its critical point, which is not a colloidal dispersion of particles.

Therefore, a colloid containing small droplets of liquid or particles of solid dispersed in a gas is known as an Aerosol.