Which of the following statements is true about an electrostatic voltmeter?

The statement that an electrostatic voltmeter measures voltage without needing external power is correct. An electrostatic voltmeter operates based on the principles of electrostatics, using an electrically charged element to detect voltage levels. It does this without drawing any current from the circuit being measured, which means it does not require external power to function. This feature makes it particularly useful for measuring voltage in high-impedance situations where loading effects need to be minimized.

Regarding the other statements: an electrostatic voltmeter does not require a magnetic field to operate, nor is it designed to measure current. Additionally, while it can be used in various applications, high-frequency measurement is typically the domain of other types of instruments, as electrostatic voltmeters are not specifically designed for that purpose.