Ampere's Law, Cylinders, Solenoid & Toroid question from MHT CET 2021 (22 Sep, Shift 2)

Answer: (b)

1. Inside a toroid $B = \mu_0 n I$, where $n$ is the number of turns per unit length.
2. With the same winding density $n$, $B$ does not depend on the radius, so $B \propto I$.
3. $B' = 3 \times 0.2 = 0.6$ mT
4. So the answer is (b) 0.6 mT.