A semi-circular loop of radius R is rotated about its straight edge which divides the space into two region one having a uniform magnetic field B and the other having no field. If initially the plane of loop is perpendicular to B (see figure) and if current flowing from O to A be taken positive, the correct plot of induced current versus time for one time period is 

As, emf induced when a loop rotates in a magnetic field is ($e=NA B\omega \sin \omega t$), which is sinusoidal function. Therefore, induced current will also be sinusoidal as shown in the figure.

The curve will be negative, because initially the flux decreases from t = 0 to T/4 and then increases from t = $\frac{T}{4}$ to $\frac{3T}{4}$.

But from t = $\frac{T}{4}$ to $\frac{3T}{4}$, there will be no induced current as there is no field on the right side. Hence, the current produced will be as shown in the figure