Correct Answer - C
Since the two coils subtend the same solid angle at O, hence area of coil `D=4` x area of coil C,
[because solid angle `=(area)/((_|_r distance)^2)`].
Therefore, radius of coil `D=2xx` radius of coil C
`:. B_D=(mu_0)/(4pi)(2piI(2r)^2)/([(2r)^2+a^2]^(3//2))`
`B_C=(mu_0)/(4pi)(2piIr^2)/([r^2+(a//2)^2]^(3//2))`
`:. (B_D)/(B_C)=(4)/((4r^2+a^2)^(3//2))xx[(4r^2+a^2)/(4)]^(3//2)=4/8=1/2`