The correct option is
Emulsion — Liquid in Liquid
Explanation
Colloids are heterogeneous mixtures where the particle size of the dispersed phase is intermediate between true solutions and suspensions. They are classified based on the physical state of the dispersed phase (solute-like component) and the dispersion medium (solvent-like component).
Option Analysis:
- Option (a) is Correct: An Emulsion is a colloidal system where both the dispersed phase and the dispersion medium are liquids. Common examples include milk, hair cream, and emulsified oils.
- Option (b) is Incorrect: A Gel consists of a liquid dispersed phase within a solid dispersion medium (e.g., jelly, cheese, butter). The pair "Solid in Liquid" refers to a Sol (e.g., paints, cell fluids).
- Option (c) is Incorrect: An Aerosol is formed when a solid or liquid is dispersed in a gas (e.g., smoke, fog, mist). The pair "Solid in Solid" refers to a Solid Sol (e.g., colored gemstones, alloys).
- Option (d) is Incorrect: A Sol represents a solid dispersed in a liquid. The pair "Gas in Liquid" refers to Foam (e.g., whipped cream, soap lather).
Key Takeaway:
Emulsion: Liquid in Liquid.
Gel: Liquid in Solid.
Sol: Solid in Liquid.
Aerosol: Solid or Liquid in Gas.