Foam is a colloidal solution of gaseous particles dispersed in liquid. Colloids are a type of mixture where one substance is dispersed evenly throughout another substance. In the case of foam, tiny bubbles of gas are dispersed throughout a liquid.

Here is a detailed explanation of why the correct answer is option 'B':

The Nature of Foam:
Foam is a type of colloid known as a foam colloid. It consists of a gas dispersed in a liquid. The gas particles are trapped within the liquid, creating bubbles that give foam its characteristic texture and appearance.

Colloidal Solutions:
A colloidal solution is a mixture in which one substance is evenly dispersed throughout another substance. The particles in a colloidal solution are larger than those in a true solution but smaller than those in a suspension. The dispersed phase in a colloidal solution can be solid, liquid, or gas.

Types of Colloids:
There are various types of colloids depending on the nature of the dispersed phase and the dispersion medium. Some common types of colloids include sols (solid dispersed in liquid), gels (liquid dispersed in solid), emulsions (liquid dispersed in liquid), and foams (gas dispersed in liquid).

Foam as a Colloid:
Foam is classified as a colloid because it consists of a dispersed phase (gas) and a dispersion medium (liquid). The gas particles are trapped within the liquid, forming bubbles. These bubbles can vary in size and are stabilized by surfactant molecules, which lower the surface tension of the liquid and prevent the bubbles from collapsing.

Characteristics of Foam:
Foam exhibits several characteristic properties of colloids, including:

1. Opacity: Foam is not transparent due to the scattering of light by the gas bubbles.
2. Stability: Foam can be relatively stable depending on the nature of the gas and the liquid. Surfactants can also enhance the stability of foam by reducing the coalescence of bubbles.
3. High surface area: Foam has a high surface area due to the presence of numerous small gas bubbles, which makes it useful in applications such as insulation and fire extinguishers.
4. Low density: Foam is less dense than the liquid it is dispersed in, as it contains a large volume of gas.

Conclusion:
In conclusion, foam is a colloidal solution of gaseous particles dispersed in liquid. The gas bubbles are trapped within the liquid and give foam its distinctive texture and properties. Understanding the nature of foam as a colloid is important in various scientific and practical applications.