Gold number of gum arabic is $0.15$. The amount of gum arabic required to protect $100 mL$ of red gold sol from coagulation by $10 mL$ of $10\%NaCl$ solution is

From the given data,

0.15 mg of lyophilic solid needed in 10 ml sol, so 100 ml would need 1.5mg

Therefore, 1.5mg of gold is needed to protect 100ml of red gold

Hence the correct option is (D).