$4I^-+Hg^{2+}\rightarrow HgI_4^{2-}$;1 mole each of $Hg^{2+}$ and $I^-$ will form _________ mole $HgI_4^{2-}$

Question

$(a)\;1mole\qquad(b)\;0.5mole\qquad(c)\;0.25mole\qquad(d)\;2mole$