The correct answer is option 4 - Ionosphere.

Radio waves are electromagnetic waves that can travel through space or through a medium, such as the Earth`s atmosphere. When radio waves encounter the ionosphere, a layer of the Earth`s atmosphere, they can be reflected back towards the Earth`s surface.

The ionosphere is a region of the Earth`s atmosphere that contains a high concentration of ions and free electrons. When radio waves pass through the ionosphere, they interact with these charged particles and can be reflected or refracted.

Option 1 - Troposphere: The troposphere is the lowest layer of the Earth`s atmosphere, where weather occurs. It does not play a significant role in reflecting radio waves.

Option 2 - Exosphere: The exosphere is the outermost layer of the Earth`s atmosphere. It is composed of very thin air and is unlikely to reflect radio waves effectively.

Option 3 - Stratosphere: The stratosphere is the layer of the Earth`s atmosphere above the troposphere. While it contains some ions and charged particles, it is not the primary layer responsible for reflecting radio waves.

Overall, it is the ionosphere that acts as a reflecting medium for radio waves, making option 4 the correct answer.