1. Can I measure Vaverage with a DMM?
Yes you can, if your DMM is a trueRMS type (the manual will tell you this & it is often printed on the meter). Strictly speaking this is RMS the average is a slightly different thing.
2. If so why Vaverage must be measured with VAC and not VDC, since the wave is half rectified
Your circuit doesn't produce DC as there is no smoothing capacitor. The diode just passes one half of the AC wave.
For DC the voltage doesn't vary so the average and peak values are the same.
3. How can I respond to d).
Use maths
For a sinewave
VRMS = 1/(sqroot 2) * Vpeak
Or approx 0.707 * Vpeak
You are only using half the waveform so divide this by 2.