C

To find the magnetic moment associated with the angular momentum of an electron, we use the formula for the magnetic moment \( \mu \) in terms of angular momentum \( L \):
\( \mu = \frac{eL}{2m} \)
Here, \( e \) is the charge of the electron, \( L \) is the angular momentum, and \( m \) is the mass of the electron.
Given that the angular momentum of the electron is \( L = j \), thus substituting this into the formula results in:
\( \mu = \frac{ej}{2m} \)
Therefore, the magnetic moment can be expressed as \( eJ/2m \), which corresponds to option C.