Self inductance or coefficient of self induction
Whenever an electric flowing through a coil changes, the magnetic flux linked with the circuit also changes. And an induced emf is produced in the circuit. This phenomenon is called self-induction. The magnetic flux linked with the coil `(phi)` is proportional to the current (I) flowing in te circuit i.e. `phi propto I`.
The self-inductance of the coil is

Options

A. `(mu_0N^2A)/(2r)`
B. `(mu_0NA)/(r^2)`
C. `(2r)/(mu_0N^2A)`
D. `r^2/(mu_0NA)`