Correct Answer - b
Field at face `ABCD= -E_(0)x_(0)hat(i)`
Flux over the face `ABCD = -E_(0)x_(0)a^(2)`
The negative sign arises as the field is directed into the cube.
Field at face `EFGH= E_(0)(x_(0)+a)hat(i)`
Flux over the face `EFGH= E_(0)(x_(0)+a)a^(2)`
Flux over the other four faces is zero as the field is parallel to the surfaces.
Total flux over the cube `=E_(0)a^(3)=(1)/(epsilon_(0))q`.
Where `q` si the total charge under the cube
`:. q= epsilon_(0)E_(0)a^(3)`.