Dynamic resistance of a p-n junction diode
how do we decide which two points to take to calculate ΔV and ΔI




Any two points on the black line marked linear approximation will do. For instance you could take 0.7 volts, 0.015 amps and 0.75 volts, 0.06 amps.

\$\Delta V\$ is 50 mV and \$\Delta I\$ is 45 mA hence R is about 1.1 ohms. Any two points would give the same answer.