Which of the following are the least reliable bands for long-distance communications during periods of low solar activity?
Why Ionospheric refraction depends on how strongly the sun ionizes the F region. During solar minimum the solar flux and sunspot count are low, so the maximum usable frequency often stays below about 20 MHz. That leaves the highest HF bands, 15, 12 and 10 meters, frequently dead for skip because signals punch through the F layer into space instead of bending back.
Watch out The low bands like 80 and 160 meters actually improve at solar minimum, since less solar activity means less D-layer absorption and quieter geomagnetic conditions; 30 and 20 meters usually still open daily even in a weak cycle.
Low sun, low MUF: the high bands (15/12/10) fade first, the low bands get better.