$\begin{array}{l}\text{Here,\hspace{0.17em}\hspace{0.17em}}\\ \Rightarrow \sqrt{\text{25}\times \text{14}-\text{6}\times {\text{7+(4)}}^{\text{?}}}\text{= 18}\\ \\ \Rightarrow \sqrt{\text{350}-{\text{42 + (4)}}^{\text{?}}}\text{= 18.}\\ \\ \text{On taking squares both sides, we get}\\ \\ \Rightarrow \text{\hspace{0.17em}350}-{\text{42 + (4)}}^{\text{?}}\text{= 324}\\ \\ \Rightarrow {\text{ (4)}}^{\text{?}}\text{= 16 }\Rightarrow {\text{ (4)}}^{\text{?}}{\text{= (4)}}^{\text{2}}\Rightarrow \text{ ? = 2}\end{array}$

Hence, Option D is correct.