A measure of the highest frequency that is useful for ionospheric propagation between two points on Earth
A count of sunspots that is adjusted for solar emissions
Another name for the American sunspot number
A measure of solar radiation with a wavelength of 10.7 centimeters✓Correct answer
Explanation
The solar flux index is a direct radiometric measurement of solar noise power at 10.7 cm wavelength, which is 2800 MHz, and it is reported in solar flux units. It correlates with EUV output and hence with F region ionisation, but it is a radio measurement, not an adjusted or renamed sunspot count.