What is the maximum altitude of the ionosphere?

The correct choice indicates that the maximum altitude of the ionosphere reaches approximately 250 miles above the Earth's surface. The ionosphere is a region of the Earth's upper atmosphere, situated approximately between 30 miles (48 kilometers) and 250 miles (400 kilometers) in altitude. This layer is ionized by solar radiation, which affects radio wave propagation and communication systems.

The significance of the ionosphere lies in its role in reflecting and refracting radio waves, which is essential for long-distance communication. The upper boundary at about 250 miles is where the ionization becomes less dense and the effects of the Earth's atmosphere diminish, marking the transition into outer space. Understanding the properties and extent of the ionosphere is crucial for fields ranging from telecommunications to atmospheric science.