What is the consequence of the low ionization in the D layer during high-frequency propagation?

The D layer of the ionosphere plays a significant role in high-frequency radio wave propagation, particularly during the daytime when it is more ionized due to solar radiation. However, the level of ionization in the D layer can be quite low during certain conditions, which impacts the behavior of radio waves.

When ionization is low, the D layer does not reflect high-frequency signals effectively. This limitation means that only a restricted range of frequencies can be transmitted without severe signal loss. Consequently, the ability to utilize high-frequency radio waves is diminished, impacting communication systems that rely on these frequencies.

This is why the correct choice reflects the fact that low ionization in the D layer results in a limited frequency range for effective transmission, as higher frequencies may just pass through without proper reflection back to the earth's surface. Thus, the consequences of low ionization culminate in a narrower scope of frequencies being available for reliable high-frequency communication.