Gold number of haemoglobin is 0.03. Hence 100 ml of gold sol will require haemoglobin in what quantity so that gold is not coagulated by 10ml 10% NaCl solution.

The correct option is C. 0.3 mg

The gold number is the minimum amount of protective colloid in milligrams which prevents a colour change from red to the violet of 10ml gold sol by the addition of 1 ml of 10%NaCl solution.

10 ml of sol needs 0.03 mg of haemoglobin (as per gold no.)
$\therefore$ 100 ml of sol would need $=100ml\times \frac{0.03mg}{10ml}$
=0.30 mg haemoglobin.