Q = C * V
Q = charge on the capacitor plates (each)
V = voltage across the plates
C = capacitance of the capacitor
C = ε A / d , Where A = area of each of the plates
d = distance between the plates.
ε = permittivity of the medium in between the plates.
If the distance d increases, then the capacitance C decreases. As a consequence, the charge on the capacitance is not affected. So Voltage difference between the plates increases.
Electrostatic energy = 1/2 C V² = 1/2 Q V = Q² / 2 C
So as the capacitance decreases, the electrostatic energy increases. The work done in moving the capacitor plates against electrostatic attraction, is stored in the capacitor as electrostatic potential energy.
So, b and d are true.