Option 1: Solid in solid - This refers to a solid dispersion in another solid, such as alloys. This is not the correct answer for smoke.
Option 2: Liquid in solid - This refers to a liquid dispersed in a solid matrix, like some types of gels. This is not the correct answer for smoke.
Option 3: Solid in liquid - This describes a suspension, where solid particles are dispersed in a liquid. This is not applicable to smoke.
Option 4: Solid in gas - This is the correct answer. Smoke consists of fine solid particles (like soot) dispersed in a gas, which is what makes it a colloidal solution of solid in gas.
Explanation:
Smoke is a colloidal solution where solid particles (such as soot) are dispersed in a gas (air). This is why it is categorized as a solid in gas colloid.