Explanation: The time constant (τ) of a resistor-capacitor (RC) circuit, crucial for understanding filter and timing characteristics in radio electronics, is calculated by the product of the resistance (R) and the capacitance (C). The formula is τ = R × C. For the given values: R = 470 kilohms = 470,000 ohms C = 100 microfarads = 0.0001 farads Multiplying these values: τ = 470,000 ohms × 0.0001 farads = 47 seconds. It is essential to convert kilohms to ohms and microfarads to farads before calculation to ensure the result is correctly expressed in seconds. The other options are incorrect due to errors in magnitude or unit conversion during the calculation.