Step 1: Understanding the relation for magnetic moment.
The magnetic moment of lanthanide ions is typically given by the relation:
\[
\mu = g \sqrt{J(J+1)}
\]
Where \( J \) is the total angular momentum quantum number, and \( g \) is the g-factor.
Step 2: Conclusion.
Thus, the correct relation is \( \mu = g \sqrt{J(J+1)} \).