If a current in a circular loop is equivalently substituted by a...

The magnetic moment of a circular loop carrying current $I$ and having area $A$ is given by $M = IA$.

When this loop is equivalently substituted by a magnetic dipole with pole strength $m$ and separation distance $d$, its magnetic moment is given by $M = md$.

Equating the two expressions for magnetic moment, we get $md = IA$.

For a given current loop, the current $I$ and area $A$ are constant. Therefore, the product $md$ must be constant.