When a diode exhibits saturated current, its dynamic plate...

The dynamic plate resistance of a diode is defined as the ratio of a small change in plate voltage to the corresponding small change in plate current, given by $r_p = \dfrac{\Delta V_p}{\Delta I_p}$.

When the diode exhibits saturated current, the plate current becomes constant and independent of the plate voltage. This means that for any change in plate voltage $\Delta V_p$, the change in plate current is zero ($\Delta I_p = 0$).

Substituting this into the formula gives $r_p = \dfrac{\Delta V_p}{0} = \infty$.

Thus, the dynamic plate resistance is infinity.