E3C10

What does the 304A solar parameter measure?

1. AThe ratio of X-ray flux to radio flux, correlated to sunspot number
2. BUV emissions at 304 angstroms, correlated to the solar flux index
3. CThe solar wind velocity at an angle of 304 degrees from the solar equator, correlated to geomagnetic storms
4. DThe solar emission at 304 GHz, correlated to X-ray flare levels

Pick an answer to check it.