Which of the following is another name for gold number ?

$(a)\;Solid\;number\qquad(b)\;Milon\;number\qquad(c)\;Congo\;rubin\;number\qquad(d)\;Avogadro's\;number$

Explanation :
Congo Rubin number was suggested by Ostwald and is expressed as the minimum amount of the protective colloid in milligrams that prevents the colour change of 100 mL of $\;0.01 \%\;$ congo rubin dye to which 0.16 gram equivalent of Kcl is added .