$\begin{array}{l}\text{Here,}\\ {\text{\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}(12)}}^{\text{2}}{\text{+ (?)}}^{\text{2}}{\text{+ (7)}}^{\text{2}}\text{= 274}\\ \\ \Rightarrow {\text{ \hspace{0.17em}144 + (?)}}^{\text{2}}\text{+ 49 = 274}\\ \\ \Rightarrow {\text{ \hspace{0.17em}\hspace{0.17em}(?)}}^{\text{2}}\text{+ 193 = 274}\\ \\ \Rightarrow {\text{ \hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}(?)}}^{\text{2}}\text{= 274}-\text{193}\\ \\ \Rightarrow {\text{ \hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}(?)}}^{\text{2}}\text{= 81}\\ \\ \text{[on taking square root both sides]}\\ \\ \Rightarrow \text{ ?=}\sqrt{\text{81}}\text{=9}\end{array}$

Hence Option D is correct.