The Mutual inductance $M_{12}$ of coil 1 with respect to coil 2-
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) - is same as $M_{21}$ of coil 2 with respect to coil 1.
$M_{12}=M_{21}=μ_0n_1n_2\pi r^2$
∴ Mutual inductance is same in the both coil.