For the forward biased diode characteristics shown in the figure, the dynamic resistance at I_D = 3 mA will be ……… $\mathrm{\Omega }$.

We know that,

$\begin{array}{c}\text{ Dynamic resistance }=\frac{\mathrm{\Delta V}}{\mathrm{\Delta l}}\\ {\mathrm{R}}_{\mathrm{d}}=\frac{0.75-0.65}{(5-1)\times {10}^{-3}}\left(\text{ at }{\mathrm{I}}_{\mathrm{D}}=3\mathrm{mA}\right.\text{ in the graph) }\\ {\mathrm{R}}_{\mathrm{d}}=\frac{0.10}{4\times {10}^{-3}}\Rightarrow {\mathrm{R}}_{\mathrm{d}}=\frac{0.1\times {10}^3}{4}\\ {\mathrm{R}}_{\mathrm{d}}=\frac{100}{4}=25\mathrm{\Omega }\end{array}$

$\therefore 25\mathrm{\Omega }\text{ is the required value of dynamic resistance at }{\mathrm{I}}_{\mathrm{D}}=3\mathrm{mA}\text{. }$