The correct answer is: A. a few volts only.
A Schering bridge is a four-arm bridge circuit used to measure the capacitance and dissipation factor of capacitors. The detector in a Schering bridge is typically a high-impedance voltmeter, which means that it draws very little current. This low current draw prevents the detector from significantly altering the voltage across the capacitor being measured.
The potential of the detector above earth potential is therefore very small, typically a few volts only. This is necessary to ensure that the measurement is accurate. If the detector were at a higher potential, it would draw more current and this would affect the measurement.
The other options are incorrect because they are too high. A potential of 1 kV, 5 kV, or 10 kV would significantly affect the measurement and would not be accurate.