In this exercise we are dealing with series RLC circuit in alternate synusoidal current.At the resonant frequency. inductive and capacitve reactance have same magnitude and opposite phase, they cancel eachother and the circuit is purely resistive.
this particular condition, the current flowing in the circuit in phase with the voltage applied.
Under the resonant the inductive reactance prevails on the capacitive reactance; in conditions the circuit behaviour is capacitive and the current the voltage applied of an angle that approaches to when fhe frequency becomes very low.
On the contrary, the frequency overcomes the resonant point, the circuit becomes and the applied voltage leads the current.