A point charge $+q$ is placed at the centre of a cube. What is the electric flux through one face of the cube?

**Total flux** through the closed cube (by Gauss's law): $\Phi_{\text{total}} = \dfrac{q_{\text{enclosed}}}{\varepsilon_0} = \dfrac{q}{\varepsilon_0}$. **By symmetry** (charge exactly at centre), each of the six identical faces receives an equal share: $\Phi_{\text{face}} = \dfrac{1}{6} \cdot \dfrac{q}{\varepsilon_0} = \dfrac{q}{6\varepsilon_0}$. **Why option A is tempting.** It's the *total* flux through the entire closed surface — the question asks about a single face. The symmetry argument is the key step; it only works because the charge sits at the geometric centre.