Here,

$\begin{array}{l}\Rightarrow 7^{2.3}\times {\left(7\right)}^{2\times 4.7}\times {(7\times 9)}^{3.4}\times {\left(9^2\right)}^{5.85}={63}^{?}\\ \\ \Rightarrow {\left(7\right)}^{\text{2.3+9.4 +3.4}}\times {\left(\text{9}\right)}^{3.4+11.70}={63}^{?}\\ \\ \Rightarrow 7^{\text{1.51}}\times {\text{9}}^{\text{15.1}}\text{=}{\left(63\right)}^{\text{?}}\\ \\ \Rightarrow {\left(63\right)}^{15.1}={\left(63\right)}^{?}\\ \\ \Rightarrow ?=15.1\end{array}$

Hence, option B is correct.