The correct answer is D) thermosphere. Here's why: Ionosphere: The reflection of radio waves is primarily the function of the ionosphere. The ionosphere is not a distinct layer in the same way as the troposphere or stratosphere, but rather a region within the upper atmosphere (primarily within the thermosphere and extending into the mesosphere) where gases are ionized by solar radiation. Layers of the lonosphere: The ionosphere consists of several ionized layers: D, E, F1, and F2. D-layer: This is the lowest layer. It absorbs high-frequency (HF) waves during the day and largely disappears at night. E-layer: This layer reflects some HF waves during the day but weakens considerably at night. F-layer (F1 and F2 merging into F-layer at night): These are the highest and most intensely ionized layers. During the day, they are distinct F1 and F2 layers. At night, the D and E layers largely dissipate due to the absence of solar radiation, and the F1 and F2 layers merge into a single, strong F-layer. This strong F-layer at night is highly efficient at reflecting high-frequency (HF) radio waves back to Earth, enabling long-distance radio communication (skywave propagation). "Particularly at night": This phrase is key. At night, the lower D and E layers fade, reducing absorption and allowing HF waves to reach the higher F-layer of the ionosphere (within the thermosphere), where they are reflected much more effectively. Therefore, the thermosphere, which contains the F-layer of the ionosphere, is responsible for efficiently reflecting high-frequency waves, particularly at night.