Given:
r1=15 cm; N=500; I=0.8 A
Side of cross-section, a=5 cm
The outer radius of the toroid will be,
r2=r1+a=15+5=20 cm
The magnetic field just inside the outer radius of the toroid will be,
B=μ0NI2πr2
B=4π×10−7×500×0.82π×20×10−2
∴B=4×10−4 T
Hence, option (C) is the correct answer.