Hello student,

You have not mentioned the diode type, so we are taking it as a Germanium diode.

Now, Forward Bias Voltage / Vd = 0.25  Volts

Reverse Saturation Current / Is = 1.2 uA

Temperature = 27'C = 300'K

AC Resistance = Rd = Vd/Id

So, first of all, we have to find Id.

Now we know that, Id = Is . ( e^Vd/nVt - 1 )

Hence put the values in this eqn, so we get Id = 0.018 A

now, Rd = 0.25/0.018 = 13.88 ohm

Hope it helps.