The sensitivity of a voltmeter is defined as the change in the instrument's reading per unit change in the voltage being measured.

In this case, the sensitivity of the PMMC voltmeter can be calculated using the following formula:

Sensitivity = Full-scale deflection current / Full-scale voltage

The full-scale deflection current can be calculated using Ohm's Law as:

I = V / (Rm + Rs)

Where V is the full-scale voltage, Rm is the internal resistance of the voltmeter, and Rs is the series resistance.

Substituting the given values, we get:

I = 250 V / (400 kΩ + 100 kΩ) = 0.5 mA

Now, substituting the calculated values of full-scale deflection current and full-scale voltage in the sensitivity formula, we get:

Sensitivity = 0.5 mA / 250 V = 0.002 Ω/V

Therefore, the sensitivity of the PMMC voltmeter is 0.002 Ω/V or 2000 Ohms/Volts.