If the current through a solenoid changes with time electromagnetic induction takes place in the solenoid. This is known as self-induction. In general, for a current I, the induced emf in the coil is `e=-L(dI)/(dt)`.
L is the self-inductance of the solenoid. On the other hand, such change in the current in a solenoid can produce electromagnetic induction in another adjacent solenoid. The induced emf in the other solenoid `e=-M(dI)/(dt)`, M is called the mutual inductance of the solenoids.
If `L_(1)` and `L_(2)` are the self-inductance of the adjacent coils then their mutual inductance `M=ksqrt(L_(1)L_(2))`. If the magnetic flux produced by the current in one coil is totally linked with the other coil then k = 1.

Answer

C