21? × 216.5 = 2112.4

Answer: Option C

$$\begin{array}{rl}& 21^{?}\times 21^{6.5}=21^{12.4}.....\left[\because a^m\times a^n=a^{m+n}\right]\\ & \Rightarrow 21^{?+6.5}=21^{12.4}\\ & \Rightarrow ?+6.5=12.4\\ & \Rightarrow ?=12.4-6.5\\ & \Rightarrow ?=5.9\end{array}$$