A coil having N turns and area A rotates in a uniform magnetic field B with angular velocity ω,
Then the peak value of the emf (Vmax) induced in the coil is given by (Vmax) = NABω
Now the fact is that the area of the coil in the 2nd case is 4A since the radius is doubled.
But the number of turns will be N/2 in the second case since the length of the wire required for a turn is doubled.
If V1 and V2 are the peak values of the emf in the two cases,
We have V1/V2 = 100 /V2 = NABω / [(N/2)×(4A)Bω]
Or, 100 /V2 = ½
V2 = 200 volt.