Answer
(a) remain the same
(b) increase
(c) increase
(d) increase
Work Step by Step
(a) We know that the electric field between the plates remains the same in the presence of the dielectric as the potential difference and the distance of the separation are the same.
(b) Since more charge is required to produce the same electric field in the presence of the polarizing effect of the dielectric, thus the charge on the plates increases.
(c) We know that the capacitance of the capacitor increases in the presence of the dielectric because the capacitor stores more charge for the constant potential difference.
(d) We know that $U=\frac{1}{2}CV^2$. As in part (b) and (c), the dielectric increases the charge and the capacitance, thus the energy stored in the capacitor also increases.
