*Question G4D04:
What does an S meter measure?*  

*Correct Answer is  C: Received signal strength*  

### AI Response: (may contain errors)  

The correct answer to the question "What does an S meter measure?" is:

**C: Received signal strength**

### Explanation of the Correct Answer:
- An S meter is a device found on many receivers that measures the strength of the received radio signal. It provides a relative indication of the signal level, typically expressed in S-units (S1 to S9), where each S-unit represents a specific increase in signal strength.

### Why the Other Choices are Incorrect:
- **A: Carrier suppression**  
  - Carrier suppression refers to the reduction of the carrier signal in a modulated transmission, which is not what an S meter measures. An S meter does not provide information about the presence or absence of the carrier.

- **B: Impedance**  
  - Impedance is a measure of how much a circuit resists the flow of electric current, typically measured in ohms. An S meter does not measure impedance; it measures signal strength.

- **D: Transmitter power output**  
  - Transmitter power output refers to the amount of power a transmitter emits, usually measured in watts. An S meter does not measure this; it measures the strength of the signal received at the receiver, not the power output of the transmitter.

### Useful Information:
- The S meter typically provides a logarithmic scale of signal strength, where:
  - Each S-unit increase (e.g., from S5 to S6) represents a tenfold increase in signal strength.
  
This understanding of the S meter is crucial for evaluating the quality of received signals during amateur radio operations.