assuming sinusoidal waves, and since there is no apparent load, all three circuits act as peak detectors.
If Veff is Vrms, then Vpeak = sqrt(2) * Veff or Vpeak = sqrt(2) * 20,5 V = 28.99 V
remove the 0.7 V diode drop yields 28.3 V
Since the drawing asks for Uab, point b is the reference
the top drawing would measure +28.3 V
the middle and bottom drawings would measure -28.3V because the diode is in the opposite direction,
making b the more positive location