The correct answer is D.
A vector voltmeter is a device that measures the amplitude and phase difference of two signals. It does this by measuring the amplitude of the signals at two points and then using the phase difference to calculate the vector sum of the two signals.
Option A is incorrect because a vector voltmeter does not work on the principle of complex variation. Complex variation is a mathematical technique that is used to analyze signals that are composed of multiple frequencies. A vector voltmeter, on the other hand, only measures the amplitude and phase difference of two signals.
Option B is incorrect because a vector voltmeter does not measure the response of linear ramp voltage. A linear ramp voltage is a type of voltage that increases linearly with time. A vector voltmeter, on the other hand, measures the amplitude and phase difference of two signals.
Option C is incorrect because a vector voltmeter is not the same as a digital meter. A digital meter is a device that measures the amplitude of a signal and then displays the result as a number. A vector voltmeter, on the other hand, measures the amplitude and phase difference of two signals and then displays the result as a vector.