*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 measures the short-term stability of Earth's geomagnetic field, which is why choice B is correct. Here's a breakdown of the choices:

1. **Correct Answer: B - The short-term stability of Earth’s geomagnetic field**
   - The K-index is a scale that quantifies disturbances in the horizontal component of Earth's magnetic field. It is used to assess geomagnetic activity over a 3-hour period. A higher K-index indicates more geomagnetic disturbance, which can affect radio propagation.

2. **Why the other choices are incorrect:**
   - **A: The relative position of sunspots on the surface of the Sun**
     - The K-index does not measure sunspot positions. Sunspots are related to solar activity but are tracked using different indices, such as the Sunspot Number.
   - **C: The short-term stability of the Sun’s magnetic field**
     - The K-index is specifically related to Earth's geomagnetic field, not the Sun's magnetic field. Solar magnetic activity is often measured by other indices, like the Solar Flux Index.
   - **D: The solar radio flux at Boulder, Colorado**
     - The solar radio flux is measured by the Solar Flux Index, not the K-index. The Solar Flux Index is a measure of solar radio emissions at a wavelength of 10.7 cm, which is used to assess solar activity.

In summary, the K-index is a key indicator for understanding geomagnetic conditions on Earth, which can impact radio communications and other technologies.