Concept:The ionosphere is a region of the atmosphere where solar radiation creates ions. It is a sub-layer of the thermosphere.Explanation:The ionosphere lies between 80 km and 400 km altitude, placing it within the thermosphere. Here, high-energy ultraviolet and X-rays from the Sun ionize atoms, producing free electrons and ions. This charged layer reflects radio waves, allowing long-distance communication. The thermosphere, which contains the ionosphere, also exhibits temperatures rising sharply with altitude due to solar absorption. Auroras, caused by solar wind interaction, appear in this layer. The troposphere (weather layer), stratosphere (ozone layer), and exosphere (outermost) do not contain the ionosphere.Answer:Thermosphere (Option C).