Correct Option: D

The corona is the outermost layer of the Sun, starting at about 1300 miles (2100 km) above the solar surface (the photo-sphere). The temperature in the corona is 500,000 K (900,000 degrees F, 500,000 degrees C) or more, up to a few million K. The corona cannot be seen with the naked eye except during a total solar eclipse. The corona does not have an upper limit.