Assertion: Gold sol prepared by the reduction method is negatively charged.
Reason: The negative charge is due to the adsorption of negatively charged $ {\text{AuO}}_2^ - $ ions dispersed phase particles.
A. Both assertion and reason are correct and reason is the correct explanation for assertion
B. Both assertion and reason are correct and reason is not the correct explanation for assertion
C. Assertion is correct but reason is incorrect
D. Assertion is incorrect but reason is correct

Hint: A colloid is a heterogeneous mixture in which a phase remains dispersed in the suspension. The size of colloidal particles varies from $10$ to $1000\,\buildrel _{\circ} \over {\mathrm{A}} $. Particles of colloidal solution carry a charge. The charge of the colloidal solution depends upon the change of ions adsorbed.

Complete answer:
Gold sol is a colloidal solution of gold.
Gold sol is prepared by heating chloride salt of gold with a reducing agent. By the reaction of chloride of gold with reducing agent gold atom forms.
The chemical formula of chloride of gold is ${\text{AuC}}{{\text{l}}_{\text{3}}}$ and the reducing agent is tin (II) chloride \[{\text{SnC}}{{\text{l}}_2}\]. By the reaction of ${\text{AuC}}{{\text{l}}_{\text{3}}}$ with reducing agent tin (IV) chloride \[{\text{SnC}}{{\text{l}}_2}\], gold atoms and tin (IV) tetrachloride \[{\text{SnC}}{{\text{l}}_{\text{4}}}\] forms.

The reaction is as follows:
${\text{2}}\,{\text{AuC}}{{\text{l}}_{\text{3}}}\, + \,3\,{\text{SnC}}{{\text{l}}_2}\, \to 2{\text{Au(sol)}}\,\,{\text{ + }}\,3\,{\text{SnC}}{{\text{l}}_{\text{4}}}$
The gold particles that are dispersed phase particles adsorbed negatively charge ${\text{AuO}}_2^ - $ so, all the gold particles have a negative charge$({\text{Au/AuO}}_2^ - )$.
So, it is true that the gold sol prepared by the reduction method is negatively charged and the reason for the negative charge is the adsorption of negatively charged ${\text{AuO}}_2^ - $ ions ion dispersed phase gold particles.
So, the assertion and reason both are true.

Therefore, option (A) Both assertion and reason are correct and the reason is the correct explanation for the assertion, is correct.

Note:
Colloidal solution can be negatively or positively charged. The acidic dyes, metal sol, and metallic sulphide are generally negatively charged sol. The basic dyes and metallic oxide are generally positively charged sol. The haemoglobin is also positively charged sol. All particles of the colloidal solution possess the same charge. The charge is due to selectively adsorption of ions from solution or by electron gain during electrode dispersion or may be due to the formation of a double layer.