What is primarily composed of hydrogen and helium?

The photosphere is the correct choice because it is the visible surface layer of the Sun from which light is emitted. This layer is predominantly composed of hydrogen and helium, which are the main components of the Sun itself. Approximately 74% of the Sun's mass is hydrogen, and about 24% is helium, making it the primary constituent of its photosphere.

In contrast, the ionosphere is a layer of the Earth’s atmosphere that is ionized by solar and cosmic radiation, primarily containing oxygen, nitrogen, and free electrons, rather than being composed mostly of the two lightest elements. The chromosphere, which lies above the photosphere, also consists mainly of hydrogen and helium but is not as representative of the entire solar mass as the photosphere itself. The magnetosphere refers to the region around an astronomical body dominated by its magnetic field, which is not about compositional elements like hydrogen and helium.