Answer
$(a)$ The magnitude of the field is small, and its rate of change is large.
Work Step by Step
We know, the magnetic flux is given by,
$\phi=BAcos\theta$
As the plane is parallel to the field $\theta=0$
So, the flux is having the maximum amount, i.e. $\phi=BA$
Again, the average induced emf is,
$\epsilon=d(B.A)/dt$
So, even though the magnitude of the magnetic field is small, the flux will be having maximum value, as well as the average emf also having maximum value.
So, the correct option is $(a)$