The voltage is generated by the changing of the magnetic flux going through the loop. For a constant magnetic field, the flux is given as Flux = B * A. So the flux at t=0 is: Flux(0) = B * pi * r^2 = 0.5 * pi * 2^2 = 2 pi. After 0.25 s, the area is zero, so the flux will also be zero. That means the change of the flux changes from 2 pi to zero in 0.25 s. According to Wikipedia, the EMF (that is, the voltage) is given as EMF = - d Flux / dt. d Flux is nothing more than the change in flux, which is 2 pi - 0 = 2 pi. And dt is the time in which that happens, which is 0.25 - 0 = 0.25. So - d Flux / dt = 2 pi / 0.25 = 8 pi. So the EMF = -8 pi. This is you voltage.
The magnetic field point into the paper, which means the EMF calculated turns clockwise, but because of the minus sign, the actual voltage runs anticlockwise.
The magnetic field point into the paper, which means the EMF calculated turns clockwise, but because of the minus sign, the actual voltage runs anticlockwise.