Biot-Savart Law & Circular Coils question from NEET 2002 (AIPMT)

Answer: (c)

1. Same wire length: $2\pi R = n \cdot 2\pi r$, so for $n = 2$, $r = R/2$.
2. $B = \frac{\mu_0 n I}{2r} \propto \frac{n}{r}$, so $\frac{B'}{B} = \frac{2}{1}\times\frac{R}{R/2} = 4$
3. So the answer is (c) $4B$.