A DC voltmeter measures the average voltage at its terminal hence it doesn't read anything when a true AC signal is applied. In your case, with the signal shown a DC voltmeter will measure the average and that is 2.5 volts because, as Brian Drummond points out, the duty cycle is 25%. This means the correct answer is "B".
Under no circumstances is the voltage measured by a DC voltmeter 10V. An RC low pass network is very similar to how the DC voltmeter gradually acquires the average voltage of the input: -
The example shows an input square wave with peak of 1V and 50:50 duty cycle. The voltmeter (aka capacitor) gradually attains an average voltage that is 0.5 volts. Note that there is also a slight ripple but, with a conventional meter and a square wave of duty cycle 40 us, the ripple will be negligible.
So, you have either specified the question incorrectly or the answer you were given is incorrect (or I am wrong).