As we know that the formula of mutual inductance is given by in this is the area of cross-section of coil which is common and l is also common length and n₁ and n₂ are the no. of turns in the coils.

So option (a) is correct and option (d) is also correct. In above formula we can see that current is independent and also angle has no meaning hence options (b) & (c) are wrong.

Therefore, options (a) & (d) are the correct.