In a single-phase parallel RL circuit connected to an AC supply, the behavior of current and voltage can be analyzed as follows:

1. Understanding Parallel RL Circuits: A parallel RL circuit consists of a resistor (R) and an inductor (L) connected in parallel to an AC supply.
2. Voltage and Current Relationship: In AC circuits, phase relationships between voltage and current are crucial for understanding power factor and circuit behavior.
3. Inductive Nature: In an RL circuit, the inductor causes the current to lag the voltage. This is due to the inductor's property of opposing changes in current, leading to the phase difference.
4. Power Factor Explanation:
   • The power factor (PF) is a measure of how effectively electrical power is being used. It is calculated as the cosine of the phase angle (\theta) between the current and voltage: PF = \cos(\theta).
   • In an ideal inductor or an RL circuit dominated by inductance, the current lags the voltage, making the phase angle positive.
5. Evaluating the Options:
   • Power factor is unity: This occurs when the phase angle is zero, which is not the case in an RL circuit due to induction.
   • Current Leads Voltage: Typically happens in a capacitive circuit, not inductive.
   • Power Factor is Leading: Leading power factor is associated with capacitive circuits.
   • Current Lags Voltage: This is correct in an inductive circuit like a parallel RL circuit.

Therefore, the correct answer is: Current Lags Voltage.

This concept is fundamental in understanding alternating current (AC) circuits, particularly when dealing with inductive components.