$\begin{array}{l}\text{Here, }\\ \Rightarrow \text{2}\sqrt{\text{392}}-\text{21 + (}\sqrt{\text{8}}-{\text{7)}}^{\text{2}}{\text{= (?)}}^{\text{2}}\\ \\ \Rightarrow \text{ 2}\sqrt{\text{392}}-\text{21 + 8 + 49}-\text{14}\sqrt{\text{8}}{\text{ = (?)}}^{\text{2}}\\ \\ \Rightarrow \sqrt{\text{4}\times \text{392}}-\text{21 + 8+ 49}-\sqrt{\text{196}\times \text{8}}\\ {\text{ \hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}= (?)}}^{\text{2}}\\ \\ \Rightarrow \sqrt{\text{1568}}-\text{21 + 8 + 49}-\sqrt{\text{1568}}{\text{ = (?)}}^{\text{2}}\\ \\ \Rightarrow {\text{(?)}}^{\text{2}}\text{= 36 }\Rightarrow \text{\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}? = 6}\end{array}$

Hence, Option E is correct.