Chapter 19 - Magnetism - Learning Path Questions and Exercises - Exercises - Page 688: 20

(c) 20 cm above the center of the loop.

The magnetic field on the central axis of circular coil of N loops is $B=\frac{mu\times NIr^{2}}{2(r^{2}+x^{2})^{\frac{3}{2}}}$, x being the distance from the center. So the field strength is maximum at the center (x=0) and reduces as one goes away. Thus in this case magnetic strength will be minimum at 20cm above the center of the loop.