A solenoid having area \( A \) length \( 'l' \) is filled with a material having relative permeability \( 2 \). The magnetic energy stored in the solenoid is:
\( \frac{B^{2} A l}{4\mu_{0}} \)
\( B^{2} A l \)
\( \frac{B^{2} A l}{\mu_{0}} \)
\( \frac{B^{2} A l}{2\mu_{0}} \)