Which region of the ionosphere is the least useful for long-distance radio-wave propagation?

The D region is the lowest layer of the ionosphere and is relatively dense. Instead of refracting (bending) high-frequency (HF) radio waves back to Earth like the higher layers do, the D region absorbs them. This absorption happens because the dense atmosphere at this lower altitude causes excited electrons to collide frequently with neutral gas molecules, dissipating the radio wave's energy as heat. Because it acts as a sponge for lower HF frequencies, the D region is actually a hindrance rather than an aid to long-distance skywave propagation. The E and F regions, in contrast, are the primary layers used for bouncing signals over the horizon.