Answer: C. 19.2 V
In an RC series circuit, 12 V (RMS) is measured across the resistor, and 15 V (RMS) is measured across the capacitor; the RMS source voltage is 19.2 V.
Given in the RC series circuit
R.M.S voltage across the resistor VR=12 V
R.M.S voltage across the capacitor VC=15 V
In the RC series circuit, the RMS source voltage Vs=√(VR2+VC2)