Concept:
A colloidal system is classified based on dispersed phase and dispersion medium.
Step 1: Check each option.
Option (A): Sol -- Butter
A sol is:
Solid dispersed in liquid
Example:
Paint, starch solution
Butter is actually:
Water in oil emulsion
So, incorrect.
Option (B): Gel -- Smoke
A gel is:
Liquid dispersed in solid
Example:
Jelly, cheese
Smoke is:
Solid dispersed in gas
So, incorrect.
Option (C): Emulsion -- Milk
An emulsion is:
Liquid dispersed in liquid
Milk consists of:
Fat droplets dispersed in water
Hence, milk is an emulsion.
So, correct.
Option (D): Foam -- Paint
Foam is:
Gas dispersed in liquid
Example:
Soap lather
Paint is actually a sol.
So, incorrect.
\[
\boxed{\text{Emulsion -- Milk}}
\]
Therefore, the correct answer is:
\[
\boxed{(C)}
\]