The phase difference between the alternating current and emf is $\pi /2$. Which of the following cannot be the constituent of the circuit?

A phase difference of exactly $\pi /2$ (or $90^{\circ }$) occurs in purely reactive circuits:Pure inductor ($L$ alone): $V$ leads $I$ by $\pi /2$.Pure capacitor ($C$ alone): $I$ leads $V$ by $\pi /2$.$L-C$ circuit: The net reactance is either inductive or capacitive, so phase difference is $\pi /2$.In an $R-L$ circuit, the phase difference is ${\tan }^{-1}(X_L/R)$, which is always less than $\pi /2$ as long as $R>0$.