A circuit when connected to an AC source of $12\text{V}$ gives a current of $0.2\text{A}$. The same circuit when connected to a DC source of $12\text{V}$, gives a current of $0.4\text{A}$. The circuit is:

With DC source ($f=0$), current $I=\frac{V}{R}⟹0.4=\frac{12}{R}⟹R=30\Omega$.With AC source, current $I=\frac{V}{Z}⟹0.2=\frac{12}{Z}⟹Z=60\Omega$.Since $Z>R$, the circuit must contain components that provide reactance ($X_L$ or $X_C$).However, a capacitor blocks DC (gives zero current). Since the DC current is non-zero, it cannot be a series $RC$ or $LC$ circuit.Thus, it must contain an inductor ($LR$ series).