The magnetic moment of a current loop is $2.1 × 10^{-25} A m^2$. The magnetic field on the axis of the loop at a distance of 1.0 Å from the loop would be: (Given radius of loop negligible)

$4.2 × 10^{-2} T$

The correct answer is Option (3) → $4.2 × 10^{-2} T$

The formula for the Magnetic field B at a distance 'r' from the center of the current loop is -

$B=\frac{μ_0}{4π}.\frac{2M}{r^3}$

where,

M = Magnetic Moment = $2.1×10^{-25}$ [Given]

$∴B=\frac{μ_0}{4π}×\frac{2(2.1×10^{-25})}{(1.0×10^{-10})^3}$

$=10^{-7}×\frac{2(4.2×10^{-25})}{(1.0×10^{-10})^3}$

$=4.2 × 10^{-2} T$