The mutual inductance M12 of coil 1 with respect to coil 2

(a) increases when they are bought nearer
(b) depends on the current passing through the coils
(c) increases when one of them is rotated about an axis
(d) is the same as M21 of 2 coil with respect to coil 1

Choose the correct option

1. (a) and (b)

2. (b) and (c)

3. (c) and (d)

4. (a) and (d)

(a, d)
The mutual inductance M12 of coil increases when they are bought nearer and is the same as M21 of coil 2 with respect to coil 1. 
M21 i.e., mutual inductance of a solenoid S1 with respect to the solenoid S2 is given by
M21=μ0n1n2π21l
where signs are as usual.
Also, M12 i.e., mutual inductance of solenoid S2 with respect to the solenoid S1 is given by
                                            M21=μ0n1n2π21l
 So, we have                      M12=M21=M