The I - V characteristics of a p - n junction diode in forward bias is shown in the figure. The ratio of…

Dynamic resistance of a diode is defined as, $R=\frac{∆V}{∆i}$.

At forward bias voltage of $2 \mathrm{V}$,

$R_1=\frac{∆V}{∆i}=\frac{2.1-2}{\left(10-5\right)\times {10}^{-3}}=20 \mathrm{\Omega }$

At forward bias voltage of $4 \mathrm{V}$,

$R_2=\frac{∆V}{∆i}=\frac{4.2-4}{\left(250-200\right)\times {10}^{-3}}=4 \mathrm{\Omega }$.

Therefore,

$\frac{R_1}{R_2}=\frac{20}{4}=\frac{5}{1}$