In a black box of unknown elements (L or R or any other combination), an ac voltage$E=E_0 \sin \left(\omega t+\varphi \right)$ is applied and current in the circuit was found to be $I=I_0 \sin \left[\omega t+\varphi +\left(\pi /4\right)\right]$. Then the unknown elements in the box may be

Since current is leading emf, capacitor must be present in the circuit. Along with capacitor, there can be either (i) only resistor
or (ii) resistor and inductor if X_C > X_L

In both of the above conditions, we can obtain a phase difference of$\pi /4$ with current leading emf