D. visible surface of the Sun.
The sun's atmosphere is composed of three main layers. 1. Photosphere- the lowest layer and closest in distance from the the sun's core. It has an average temperature of 5,800 degrees Kelvin. It usually appears granulated and bubbly, like a simmering pot of water. 2.Chromosphere- extends above the photosphere to about 2000 km. Its temperature is 4,500 - 10,000 degrees Kelvin. 3. Corona - outermost layer of the sun, extending several million kilometers outward from the chromosphere into space. The average temperature averages 2 million degrees Kelvin. It is thought the sun's magnetism is the reason that the corona is so hot.