G3A12:What does the K-index measure?
Direct answer summary (2023-2027 NCVEC pool)
The official correct answer for G3A12 is The short-term stability of Earth’s geomagnetic field.
The key fact is The short-term stability of Earth’s geomagnetic field. Match that fact to the exact condition in the question; the other choices describe a different rule, band, component, or operating situation.