A

Step 1: Understand the Assertion. The assertion states that resonance in an RLC circuit occurs only when both the inductance (L) and capacitance (C) are present, which is indeed true. Resonance occurs at the frequency where the inductive and capacitive reactances are equal, resulting in the circuit having maximum current.

Step 2: Analyze the Reason. The reason states that the voltage across L and C cancel each other and the current amplitude is \frac{V_m}{R}, which is also true. At resonance, the impedance is minimized, and the voltage across the inductor and capacitor do cancel each other.

Step 3: Correlate Assertion with Reason. The reason correctly explains why resonance occurs when both L and C are present in the circuit. Therefore, both the Assertion and Reason are true, and the Reason does provide a correct explanation for the Assertion.

Therefore, the correct answer is A.