What is the phase relationship between the current through and the voltage across a series resonant circuit at resonance?
Why At resonance the inductive reactance and capacitive reactance are equal in magnitude and opposite in sign, so they cancel and leave only resistance. A purely resistive impedance produces no phase shift, so the current through the series circuit and the voltage across it rise and fall together. This zero phase angle is one of the defining tests for resonance, along with minimum impedance and maximum current in a series circuit.
Watch out The 90 degree answers describe a circuit dominated by a single reactance: voltage leads current by 90 degrees in a pure inductor, and current leads voltage by 90 degrees in a pure capacitor. At resonance neither dominates because they cancel each other.
Resonance = reactances cancel = looks like a plain resistor = zero phase angle, series or parallel.