# By which of the following methods . Gold sol is prepared ?

$(a)\;Reduction\qquad(b)\;Oxidation\qquad(c)\;Hydrolysis\qquad(d)\;Double\;decomposition$

Explanation :
The colloidal solutions of metals are obtained by reduction of their compounds .
A solution of $\;Aucl_{3}+3 Sn Cl_{2}\; \to \; 3Sncl_{4}+2 Au \;Gold\; sol$
The reaction can also be carried out with formaldehyde , tannic acid or hydrazine .
$2 Aucl_{3}+3HCHO + 3 H_{2}O \; \to \; 2 Au + 3HCOOH+6Hcl\;Gold\;sol$