Biot-Savart Law & Circular Coils question from MHT CET 2025 (22 Apr, Shift 1)

Answer: (b)

1. Straight parts PQ and RS point towards O, so they give no field at O.
2. Outer semicircle ($R_1$, clockwise): $\dfrac{\mu_0 I}{4R_1}$ into the page.
3. Inner semicircle ($R_2$, anticlockwise): $\dfrac{\mu_0 I}{4R_2}$ out of the page.
4. $B = \dfrac{\mu_0 I}{4}\left[\dfrac{1}{R_2}-\dfrac{1}{R_1}\right]$ (since $R_2 < R_1$)
5. So the answer is (b).