Which of the following statements about the D layer is true?

The statement indicating that the ionization density of the D layer is lowest compared to higher layers is accurate. The D layer, which is part of the ionosphere, is the region that lies between about 30 to 60 miles above the Earth's surface. It contains free electrons and ions that are created by solar radiation. However, the ionization characteristics of the D layer differ significantly from those of the higher layers, such as the E and F layers.

During the daytime, the presence of UV radiation from the sun causes increased ionization in the D layer, but this layer still maintains a lower ionization density than the E and F layers. As altitude increases, the ionization density generally increases due to the stronger solar radiation exposure, resulting in the E and F layers having more free electrons.

In contrast, the D layer does affect radio wave propagation, particularly at lower frequencies, but it is primarily characterized by its relatively low ionization compared to its higher counterparts. Additionally, the D layer's ionization levels are influenced by day and night cycles, which affects its thickness and strength, unlike the incorrect assertion that it remains unaffected.

Overall, option C correctly highlights the fundamental characteristic of the D layer in regards to its ionization density when compared to