On addition of $$1$$ mL of $$10\%$$ NaCl solution to $$10$$ mL gold sol in the presence of $$0.025$$ g of starch, the coagulation is just prevented. Starch has gold number

The gold number (introduced by Zsigmondy) is defined as “the minimum mass in milligrams of a protective colloid that will just prevent the coagulation of $$10 \text{ mL}$$ of a standard gold sol when $$1 \text{ mL}$$ of $$10\%$$ NaCl solution is added.”

Given data:
 Mass of starch used $$=0.025 \text{ g}$$

Convert the mass into milligrams because the definition uses mg:

$$0.025 \text{ g}=0.025\times1000=25 \text{ mg}$$

This 25 mg of starch is just sufficient to protect $$10 \text{ mL}$$ of the gold sol from coagulation. Therefore, according to the definition, the gold number of starch is:

$$\text{Gold number}=25$$

Option B which is: $$25$$