In which case racemic mixture is obtained on mixing its mirror images in 1 : 1 molar ratio?

(a) [Cr(en)3]3+ exists in d- and l-forms due to the absence of one of the symmetry element(s). So it will give racemic mixture when d- and l-forms are mixed in 1 : 1 molar ratio. (b) [Ni(DMG)2] has square planar geometry and thus has mirror plane so optically inactive.(c) cis-[Cu(Gly)2] has square planar geometry and thus have mirror plane so optically inactive.