The I - V characteristics of a p - n junction diode in forward bias...

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

At forward bias voltage of $2V$,

${R}_{1} = \dfrac{∆ V}{∆ i} = \dfrac{2 . 1 - 2}{\left(10 - 5\right) \times {10}^{- 3}} = 20Ω$

At forward bias voltage of $4V$,

${R}_{2} = \dfrac{∆ V}{∆ i} = \dfrac{4 . 2 - 4}{\left(250 - 200\right) \times {10}^{- 3}} = 4Ω$.

Therefore,

$\dfrac{{R}_{1}}{{R}_{2}} = \dfrac{20}{4} = \dfrac{5}{1}$