Correct Answer - A,D
The mutual inductance `M_2` of coil increases when they are brought nearar and is the same as `M_2` of coil `2` with respect to coil 1.
`M_12`i.e., mutual inductance of solenoid `S_1`with respect to scolenoid `S_2` given by `M_21=mu_(0)n_(1)n_(2)pir_(1)^2l`
where signs are as usual.
Also, `M_12` i.e., mutual inductance of solenoid `S_2` with respect to scolenoid `S_1`is givne by `M_21=mu_0n_1n_2pir_(1)^(2)l`
So, we have `M_12=M_21=M`