b
\(\ln \left[C r\left(N H_{3}\right)_{6}\right]^{3+},\) Cr is present as \(C r^{3+}\)

\(C r^{3}+=[A r] 3 d^{3}, 4 s^{0}\)

since, this complex has three unpaired electrons, excitation of electrons is possible and thus, it is expected that this complex will absorb visible light.