Assertion:
A colloidal sol scatters light but a true solution does not.
Reason:
The particles in a colloidal sol move slower than in a true solution.

A.Both assertion and reason are correct and the reason is the correct explanation for assertion
B.Both assertion and reason are correct but the reason is not the correct explanation for assertion
C.The assertion is correct but the reason is incorrect
D.Both assertion and reason are incorrect

Hint: In chemistry, a solution is an important term that is a mixture of two or more than two components. In the solution, solute and solvent are an important part of it. It is categorized into two-parts that is the homogeneous solution and the heterogeneous solution.

Complete step by step answer:
Let's discuss important terms of solution.
Solute - A substance that dissolves in the solvent.
Solvent - A substance that dissolves the solute.
Homogeneous solution - A solution that has a fixed ratio of solute and solvent.
Heterogeneous solution - A solution that has no fixed ratio of sample components.
Based on the properties, the solution is divided into three parts.
True solution - It is a homogeneous solution, in which solute and solvent are completely mixed.
example - salt in water
Suspension - It is a heterogeneous solution, in which particles are not completely mixed. It can be seen by eyes easily.
example - mud in the water
Colloidal - It is a heterogeneous solution, in which particles are not completely mixed. In this solution particle size is greater than the true solution and less than suspension.
example - smoke in the air, fog.
As we discuss the particle in the true solution is very small, so it does not show the Tyndall effect while the particle in the colloidal solution is larger that moves easily and shows the Tyndall effect.

 Therefore, option A is the correct answer here.

Note: The size of the true solution particle is less than $1nm$, the size of the suspension particle can be greater than $1000nm$, and the size of the colloidal particle is $1 - 1000nm$. Tyndall effect - When light scatters by particle, it is called the Tyndall effect. It was given by John Tyndall.