Two capacitors, C1 = 6 µF and C2 = 3 µF, are connected in series across a DC supply. What is the total capacitance of the combination?

Why C is correct

NEETS Mod. 2 §3-2: capacitors in series combine by the reciprocal rule. 1/Ct = 1/6 + 1/3 = 1/6 + 2/6 = 3/6 = 1/2, so Ct = 2 µF. Series connection reduces total capacitance below the smallest individual value; parallel connection would add them to 9 µF.