How does the strength of the magnetic field at the centre of a circular coil of wire depend on
(i) the radius of the coil,
(ii) the number of turns of wire in the coil, and
(iii) the strength of current flowing in the coil?

The strength of magnetic field depends upon:

(i) Magnetic field(B) is inversely proportional to the radius of the coil.
That is,
                               $\mathrm{B} \propto \frac{1}{\mathrm{r}}$
(ii) B is directly proportional to the number of turns n of the coil. For each circular turn, the direction of current is same. So, the fields due to the various turns get added up.
That is,
                                B ∝ n 

(iii) B is directly proportional to the strength of current passing the coil.
That is,
                                B ∝ I