The correct answer is: C. Zero
An ideal voltmeter is a theoretical device that has zero resistance. This means that it does not draw any current from the circuit it is measuring, and therefore does not affect the voltage reading.
A real voltmeter, on the other hand, has some finite resistance. This resistance causes it to draw a small amount of current from the circuit, which can affect the voltage reading. The higher the resistance of the voltmeter, the greater the error in the voltage reading.
For this reason, it is important to choose a voltmeter with a high input impedance. This means that the voltmeter has a high resistance to current flow, and therefore does not affect the circuit it is measuring.
The other options are incorrect because they do not describe the resistance of an ideal voltmeter. Option A is incorrect because an ideal voltmeter has zero resistance, not low resistance. Option B is incorrect because an ideal voltmeter has zero resistance, not infinite resistance. Option D is incorrect because an ideal voltmeter has zero resistance, not high resistance.