Does MgCl2 sol coagulate gold sol faster than Fe(OH)3 sol?
Option (c) is actually correct. Magnesium chloride does coagulate gold sol faster than ferric hydroxide, because ferric hydroxide does not coagulate in the first place.

Wikipedia mentions that ferric hydroxide is insoluble at $\ce{pH}=7$, so there aren't any $\ce{Fe^{3+}}$ ions to coagulate the gold sol anyway.