Question

how does the strength of magnetic field

Answer 1

c
may be the answer because the strength depends on magnetic field

Answer 2

1. As the radius decreases , the magnetic field at the centre increases .
This is because the distance of the centre from the coil decreases.

2. As the number of turns increases the magnetic field at the centre again increases
since the current moves in one direction

therefore the magnetic field produced due to the current in each turn simply adds up.