Why The solar flux index is a direct radio measurement: observatories (notably Penticton, British Columbia) measure the sun's noise power at a wavelength of 10.7 cm, which is about 2800 MHz. That energy tracks the level of solar activity that ionizes the F region, so a higher flux generally means better HF conditions and a higher MUF. Unlike sunspot counts, it is an objective instrument reading taken daily rather than a visual tally.
Watch out The choices about counting sunspots describe the sunspot number, a separate (visual) index that correlates with solar flux but is not the same measurement; the 'highest useful frequency between two points' describes the MUF.
Solar FLUX = 10.7 cm radio noise. Remember '10.7' and you have the answer.