Answer
(a) $C_1$
(b) $C_2$
Work Step by Step
(a) We know that if the capacitors are connected in series then $U=\frac{Q^2}{2C}$. This equation shows that the energy stored is inversely proportional to the capacitance. Thus, capacitor $C_1$, having a smaller capacitance, stores more energy.
(b) We know that if the capacitors are connected in parallel, then $U=\frac{1}{2}CV^2$. This equation shows that the energy stored is directly proportional to the capacitance. Thus, the capacitor having the greater capacitance stores more energy, that is $C_2.$
