What is the magnetic field on the axis of a coil of radius $r$ carrying current $I$ at a distance $R$ from the origin?

• Correct answer
  A

  $\frac{\mu_0 I r^2}{2\left(r^2+R^2\right)^{3 / 2}}$
• B

  $\frac{\mu_0 I R^2}{2\left(r^2+R^2\right)^{3 / 2}}$
• C

  $\frac{\mu_0 I}{2(r+R)}$
• D

  $\frac{\mu_0 I R}{2\left(r^2+R^2\right)^{1 / 2}}$

Hence, the correct answer is option: 'A'.