The correct answer is: B. ionosphere
The ionosphere is a region of Earth’s upper atmosphere that is ionized by solar radiation. It is important for radio propagation because it reflects radio waves back to Earth, allowing them to travel over long distances.
The troposphere is the lowest layer of Earth’s atmosphere, extending from the surface to about 12 kilometers (7.5 miles). It is the layer where most weather occurs, and it is not very important for radio propagation.
The exosphere is the outermost layer of Earth’s atmosphere, extending from about 100 kilometers (62 miles) to about 10,000 kilometers (6,200 miles). It is very thin, and it does not play a significant role in radio propagation.
The thermosphere is the layer of Earth’s atmosphere above the mesosphere, extending from about 80 kilometers (50 miles) to about 600 kilometers (370 miles). It is very hot, and it does not play a significant role in radio propagation.
In addition to the ionosphere, other factors that can affect radio propagation include the Earth’s magnetic field, the Sun’s activity, and the Earth’s topography.