Explanation

(b) When coil is open, there is no current in it, hence no flux associated with it, $ie,\phi = 0.$
Also, we know that flux linked with the coil is directly proportional to the current in the coil,
$ie,\ \ \phi \propto i$
Or $\phi = Li$
Where $L$ is proportionality constant known as self-inductance.
$\therefore\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ L = \frac{\phi}{i} = 0$
Again since$\ i$ =0, hence $R = \infty.$