Final answer:
The layer of the Sun's atmosphere that lies immediately above the visible atmosphere or photosphere is known as the Chromosphere.
Step-by-step explanation:
The Sun has several layers, and the question refers to the layer of the Sun's lower atmosphere that lies just above the visible atmosphere, known as the photosphere. Considering the options provided, the correct answer is a) Chromosphere. The chromosphere is the part of the Sun's atmosphere that is immediately above the photospheric layers. It can be observed during a total solar eclipse when the photosphere is concealed by the Moon, revealing a narrow red “streak” or “fringe”, which led to its name that originates from the Greek word for 'colored sphere'. The chromosphere is one layer of the solar atmosphere, and above it there is the transition region and then the extremely hot corona, which is noticeable during eclipses as well.