Q.Briefly explain how to find r_d and μ from the graph.

Both $r_d$ and $\mu$ are read from the JFET drain characteristics by taking small changes between neighbouring curves. …

[!TLDR]

On the drain characteristics: $r_d = \Delta V_{DS}/\Delta I_D$ along one $V_{GS}$ curve, and $\mu = \Delta V_{DS}/\Delta V_{GS}$ taken between two curves at constant $I_D$.

The drain characteristics of a JFET are a family of $I_D$–$V_{DS}$ curves, one curve for each fixed value of $V_{GS}$. Both parameters are read off these curves:

Finding $r_d$: stay on one curve, i.e. keep $V_{GS}$ constant. In the saturation (pinch off) region choose two nearby operating points, note the small change in drain-to-source voltage $\Delta V_{DS}$ and the corresponding small change in drain current $\Delta I_D$. Then

$r_d = \frac{\Delta V_{DS}}{\Delta I_D} \quad \text{(at constant } V_{GS}\text{)}$ …