which of the following colloid is an example of foam ?

Sponge 

The correct answer is option 1. Sponge.

A foam is a type of colloid, which is a mixture where one substance is dispersed in another. In the case of foam, it consists of a gas dispersed in a liquid or solid. Let's consider the examples given:

1. Sponge: A sponge is an example of a solid foam. It is a porous material with a structure that contains numerous small air pockets or gas-filled voids. The gas in this case is trapped within the solid structure of the sponge. When the sponge is wet, the liquid is also present within its pores, creating a foam-like structure. The combination of gas (air) and liquid (water) trapped in the sponge's structure gives it the characteristic properties of a foam.

2. Butter, Cheese, and Mist: These examples don't represent the foam category.

Butter and Cheese: These are more accurately described as colloids of dispersed fat globules in water or other liquids, but they don't exhibit the foam structure of dispersed gas in a liquid or solid.

Mist: Mist is an example of a colloid where tiny liquid droplets are dispersed in a gas (air), but it doesn't represent a foam, which typically involves a larger volume of gas dispersed in a liquid or solid.

In summary, a sponge is a solid foam where gas is dispersed in a solid matrix, and when wet, it may contain liquid as well. The term "foam" generally refers to a mixture where gas is dispersed in a liquid or solid, creating a structure with many gas-filled bubbles or voids.