The Mutual inductance $M_{12}$ of coil 1 with respect to coil 2-

Decreases as they are brought nearer. Depends on the current passing through the coils. Increases when one of them is rotated about an axis. is same as $M_{21}$ of coil 2 with respect to coil 1.

is same as $M_{21}$ of coil 2 with respect to coil 1.

The correct answer is Option (4) - is same as $M_{21}$ of coil 2 with respect to coil 1.