The correct answer is option 3, the Ionosphere. The Ionosphere is a layer of the Earth`s atmosphere located between about 60 to 1,000 kilometers above the Earth`s surface. It is composed of charged particles called ions, which are created by the interaction of high-energy solar radiation with neutral particles in the atmosphere.
The Ionosphere plays a crucial role in radio communication because it reflects and refracts radio waves back to Earth. This phenomenon is known as ionospheric reflection or skywave propagation. Radio waves transmitted from the Earth`s surface reach the Ionosphere and are reflected back to Earth, allowing long-distance communication.
The other options, the Troposphere, Stratosphere, and Thermosphere, do not directly contribute to radio communication. The Troposphere is closest to the Earth`s surface and is where most weather occurs. The Stratosphere is above the Troposphere and contains the ozone layer. The Thermosphere is the outermost layer and is characterized by high temperatures.