For which of the following d" configuration of octahedral complex(es), cannot exist in both high spin and low spin forms?

(P) $d^3$

(Q) $d^{\mathit{5}}$

(R) $d^{\mathit{6}}$

(S) $d^8$

Because the complex's core metal ion's outer electronic configuration is d¹ to d³, it is an inner orbital complex.

The d³ and d⁸ octahedral complex(es) cannot exist in both high spin and low spin forms.

Thus, the correct option is C.