When the diode shows saturated current, dynamic plate resistance is
We know the dynamic plate resistance is given by
Now when diode is at saturated current region the change in ∆IP will be very small or it will be near to zero, therefore dynamic resistance will at infinity.
It can’t be indeterminate as although it will saturate current but still changes in decimal places will be still be visible and hence ∆IP will not be equal to zero.