The correct answer is D. VTVM.
A voltmeter is a device used to measure electrical potential difference (voltage) between two points in an electric circuit. A multimeter is a measuring instrument that combines several measurement functions in one unit. A CRO, or cathode ray oscilloscope, is an electronic test instrument that is used to display waveforms of electrical signals. A VTVM, or vacuum tube voltmeter, is a type of voltmeter that uses vacuum tubes to amplify the voltage signal being measured.
VTVMs are the most accurate type of voltmeter because they have a very high input impedance. This means that they do not draw much current from the circuit being measured, which can cause errors in the measurement. VTVMs are also very stable and have a wide frequency response. This makes them ideal for measuring AC and DC voltages.
Multimeters are less accurate than VTVMs, but they are more versatile. They can measure a variety of electrical quantities, including voltage, current, and resistance. Multimeters are also less expensive than VTVMs.
CROs are not used for measuring voltage. They are used to display waveforms of electrical signals. CROs can be used to measure voltage, but this is not their primary function.
In conclusion, the most accurate device for measuring voltage is a VTVM.