Explanation: The time constant (τ) of a resistive-capacitive (RC) circuit is a fundamental concept in electronics, representing the product of the circuit's total resistance (R) and capacitance (C). When a DC voltage is applied to a series RC circuit, the capacitor begins to charge exponentially. After exactly one time constant (τ = R * C) has elapsed, the voltage across the capacitor's terminals will have risen to approximately 63.2% of the applied voltage. This characteristic exponential curve describes how capacitors charge and discharge, which is crucial for understanding filtering, timing circuits, and pulse shaping in radio systems. Option A is incorrect because the "natural period of oscillation" relates to resonant LC circuits, not the charging characteristic of an RC circuit. Option B, "Varactance," is not a standard electrical term or unit of time. A varactor is a voltage-variable capacitor, but "varactance" doesn't define this concept. Option D is incorrect because ohmic resistance is measured in ohms, while the time constant is a duration measured in seconds. The time constant depends on both resistance and capacitance.