What kind of solution is milk?

Suspension: In suspension, the particles settle down if kept for some time, but in milk, the fat particles do not settle, so it is not a suspension.

Colloid: milk is a colloid in which the fat particles are dispersed in water. It is liquid in liquid colloid known as an emulsion.

True solution: the solution is a homogeneous mixture of two or more substances dissolved in the solvent, in which the particle size is very small.

Alloy: it is a mixture of two or more chemical elements in which at least one is metal.

Hence the correct answer is B i.e. Colloid.