b
(b)Its coordination number will be \( 6\) because it is bonded with three bidentale ligands.
Oxi. No. of \(Cr\) in \(\mathop {{K_3}}\limits_{ - 1} [Cr\mathop {{{({C_2}{O_4})}_3}}\limits_{ - 2} ]\) is
\(x + 3( - 2) + 3( + 1) = 0\) \( \Rightarrow \,\,x = + 3\)