b
When cube is of side a and point charge \(Q\) is at the center of the cube then the total electric flux due to this charge will pass evenly through the six faces of the cube.

So, the electric flux through one face will be equalto \(1 / 6\) of the total electric flux due to this charge.

Flux through \(6\) faces \(=\frac{Q}{\epsilon_{0}}\)

\(\therefore\) Flux through \(1\) face, \(=\frac{Q}{6 \in_{0}}\)