Colloidal solution of gold prepared by different methods have different colours, because 

A colloidal solution is a mixture of dispersed phase (solute) and dispersed medium (solvent)  in which the particle size ranges between 1 nm to 1000 nm. The color of colloidal solution depends upon the size of solute particles.

Thus, colloidal solutions of gold prepared by different methods have different colors, because of the difference in the diameter of colloidal gold particles.

Hence, option (D) is correct.