Explanation: In a radio receiver, monitoring signal strength with a DC voltmeter requires converting the incoming AC radio frequency (RF) signal into a proportional DC voltage. This conversion happens in the detector and subsequent filtering stages. * **Point 4** represents the output of a detector circuit after rectification and filtering. The RF signal's amplitude (signal strength) is converted into a varying DC voltage envelope. A low-pass filter removes the RF component, leaving a relatively smooth DC voltage whose magnitude is proportional to the average strength of the incoming RF signal. This DC voltage can be measured by a DC voltmeter to indicate signal strength, often used for S-meters or Automatic Gain Control (AGC). * **Points 1, 2, and 3** are typically at earlier stages in the circuit. Point 1 is likely the input RF signal, which is an AC voltage. Points 2 and 3 might be within the detector before sufficient filtering, where the signal is either still high-frequency AC or a pulsating DC that a simple DC voltmeter would not accurately interpret as a stable signal strength measurement.