$\begin{array}{l}{\text{Here, (64)}}^{\text{3.1}}\times 8^{4.3}=8^{?}\\ \\ \text{By applying the rule of indices, we get}\\ \\ = (8^2{)}^{3.1}\times {\text{(8)}}^{\text{4.3}}{\text{= 8}}^{\text{?}}\\ \\ ={\text{\hspace{0.17em}8}}^{\text{?}}{\text{ = 8}}^{\text{6.2+4.3}}\\ \\ {\text{\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}= 8}}^{\text{10.5}}\\ \\ \Rightarrow ?=10.5\end{array}$

Hence, Option A is correct.