Assertion-Aqueous gold colloidal solution is red in colour.

Reason
The colour arises due to scattering of light.
A. Both Assertion and Reason are correct and Reason is the correct explanation for Assertion
B. Both Assertion and Reason are correct but Reason is not the correct explanation for Assertion
C. Assertion is correct nut Reason is incorrect
D. Assertion is incorrect nut Reason is correct

Hint: In colloidal mixture, the suspended particles are not small as in solution.
Complete step by step answer:
We know that, the phenomenon in which deviation of light rays from its straight path occurs due to striking an obstacle like gas molecule or dust, water vapor etc. is termed as scattering of light. Tyndall effect is due to scattering of light.
 Aqueous gold solution is red in colour because of scattering of light by colloidal particles of gold. The colour of solution depends on the particle size. The colour of finest gold sol is red. Increasing the size of particles of gold, it becomes purple, then colour becomes blue and finally the colour we observed for gold is golden yellow.

So, the correct option is A as both assertion and reasoning are true and reasoning is the correct explanation of the assertion.

Additional information:

Some of the properties of colloidal solution are:
1) Colloidal particles has size between ${\rm{1}}\;{\rm{nm}}$ to ${\rm{1000}}\;{\rm{nm}}$.
2) The nature of colloidal solution is heterogeneous
3) Colloidal solutions diffuse through filter paper
4) Colloidal solutions shows tyndall effect
5) Colloidal solutions appear translucent

Note: Students might get confused about the colour of gold. They might think that the colour of gold is yellow not red. But actually the colour of gold varies as the size increases. Fine gold sol has a red colour. The gold we commonly observe is the solid gold whose colour is yellow.