Explanation

(c)
The formula of dichlorodioxalatochromium (III) is $\left\lbrack Cr\left( {Cl}_{2} \right)(ox)_{2} \right\rbrack^{3 -}$
Primary valency of a metal (Cr)in the complex
$\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \equiv$ oxidation number of that metal
= +3
Secondary valency of chromium in complex
=coordination number
=+6
($\because$ Coordination number is the number of ligands attached to the central metal ion and oxalate ion is a bidentate ligand i.e., can coordinate at two positions)