*Question G3A12:
What does the K-index measure?*  

*Correct Answer is  B: The short-term stability of Earth’s geomagnetic field*  

### AI Response: (may contain errors)  

The K-index is a measure used in geomagnetism to assess the short-term stability of Earth's geomagnetic field. 

### Explanation of the Correct Answer:
- **Correct Answer: B** - The K-index quantifies the level of geomagnetic activity based on measurements from magnetometers. It ranges from 0 to 9, where higher values indicate greater disturbances in the geomagnetic field, often due to solar activity such as solar flares or coronal mass ejections.

### Why the Other Choices are Incorrect:
- **A: The relative position of sunspots on the surface of the Sun**  
  - This refers to solar activity but is not related to the K-index. Sunspots are monitored using different metrics, such as the sunspot number or solar cycle phases.

- **C: The short-term stability of the Sun’s magnetic field**  
  - The K-index specifically measures Earth's geomagnetic field, not the Sun's magnetic field. The Sun's magnetic activity is assessed using different indices, such as the solar magnetic field strength.

- **D: The solar radio flux at Boulder, Colorado**  
  - This option refers to solar radio emissions, which are measured using the Solar Radio Flux index (often the F10.7 index), not the K-index. The K-index does not measure radio flux.

### Summary:
- The K-index is crucial for understanding geomagnetic storms and their potential impact on radio communications and navigation systems. 

### Useful Rule of Thumb:
- A K-index of 5 or higher indicates significant geomagnetic activity, which can affect radio propagation and satellite operations.