Which of the following are the least reliable bands for long-distance communications during periods of low solar activity?

During periods of low solar activity (a solar minimum), there are very few sunspots and correspondingly lower levels of ultraviolet radiation reaching the Earth's atmosphere. This results in a weakly ionized F layer in the ionosphere. A weakly ionized F layer does not have the density required to bend or refract higher-frequency radio waves back to Earth. Instead, these higher frequencies pass straight through the ionosphere and escape into space. Because of this, the highest HF bands—15 meters, 12 meters, and 10 meters—become highly unreliable for long-distance skywave communication during a solar minimum. Hams must drop down to lower frequencies (like 40m, 80m, or 160m) which can still be refracted by the weaker ionosphere.