*Question G3A13:
What does the A-index measure?*  

*Correct Answer is  C: The long-term stability of Earth’s geomagnetic field*  

### AI Response: (may contain errors)  

The A-index is a measure of the long-term stability of Earth's geomagnetic field, specifically reflecting the level of geomagnetic activity over a period of time. 

### Explanation of the Correct Answer:
- **Correct Answer: C**  
  The A-index quantifies geomagnetic activity based on measurements from magnetometers around the world. It provides an average of the daily geomagnetic activity over a specified period, typically 24 hours. A higher A-index indicates more geomagnetic disturbances, which can affect radio communications and navigation systems.

### Explanation of Incorrect Answers:
- **A: The relative position of sunspots on the surface of the Sun**  
  This choice refers to solar phenomena but does not relate to the A-index. Sunspots are areas of reduced surface temperature on the Sun caused by magnetic activity, and their positions are tracked separately in solar studies.

- **B: The amount of polarization of the Sun’s electric field**  
  This option is not relevant to the A-index. Polarization of the Sun's electric field pertains to electromagnetic wave properties and is not measured by the A-index.

- **D: The solar radio flux at Boulder, Colorado**  
  This choice refers to the solar radio flux, which is a measure of solar radiation at specific frequencies. It is not related to the A-index, which focuses on geomagnetic activity rather than solar output.

### Summary:
- The A-index is crucial for understanding geomagnetic activity and its potential impact on radio communications.
- Remember that the A-index is distinct from solar phenomena like sunspots and solar radio flux measurements. 

This knowledge is essential for the Extra Class exam, as it helps you understand how solar and geomagnetic activities can affect amateur radio operations.