HELLO,
\(\overrightarrow{E}=(6\hat{i}+3\hat{j}+
4\hat{k})N/C\)
Cross-sectional area = \(20m^2\)
Surface is in Y-Z plane so its area vector must be along X- axis.
\(\overrightarrow{A}=(20\hat{i})m^2\)
As, we all know that flux is nothing but dot product of Electric field and Area vector.
\(\phi=\overrightarrow{E}\bullet\overrightarrow{A}\)
\(\phi =(6\hat{i}+3\hat{j}+4\hat{k})\bullet(20\hat{i})\)
\(\phi=120\frac{N}{m^2\cdot C}\)
So,Correct option is B.
I HOPE YOU WILL UNDERSTAND.