The relative position of sunspots on the surface of the Sun
The amount of polarization of the Sun’s electric field
The long-term stability of Earth’s geomagnetic field✓Correct answer
The solar radio flux at Boulder, Colorado
Explanation
The A-index is computed from the eight K values of a day converted to a linear amplitude scale and averaged, so it summarises geomagnetic conditions over a whole day rather than over three hours. That is why it is the long-term stability figure and the K-index is the short-term one.