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

Answer: D. 15 meters, 12 meters, and 10 meters

During low solar activity the ionosphere is too weak to refract the highest HF frequencies. That makes 15, 12, and 10 meters the least reliable for long-distance contacts, while the lower bands still work.

Tip: Low sunspots, high bands go quiet.