A parallel-plate capacitor with capacitance \(C_0\) is connected to a battery of potential difference \(V_0\) until fully charged, storing total charge \(Q_0\). The capacitor is then disconnected from the battery. An external force then slowly pulls the plates apart, increasing their separation from \(d\) to \(2d\). Which of the following correctly describes the change in the electric potential energy stored in the capacitor and provides the correct physical justification?