b
The expression for toroid magnetic field is given by

\(B=\mu_{0} \mu_{r}\left[\frac{N}{2 \pi r}\right] I\) \(......(I)\)

Substitute \(4 \pi \times 10^{-7}\) for \(\mu_{o}, 700\) for \(\mu_{r}, 250 \times 100\) for \(N 0.5\) for \(r\) and \(1.5\) for \(I\) in equation \((I).\)

\(B=\left(4 \pi \times 10^{-7}\right)(700)\left[\frac{250 \times 100}{2 \pi(0.5)}\right](1.5)\)

\(=\frac{1400 \times 10^{-7} \times 25 \times 10^{3} \times 1.5}{0.5}\)

\(=75 \times 1400 \times 10^{-1}\) Tesla

\(=10.5\) Tesla