What portion of the Sun is always illuminated?

The correct choice is that half of the Sun is always illuminated. This is a result of the Sun being a nearly perfect sphere and continuously generating light and heat from nuclear fusion occurring at its core. As the Sun rotates on its axis, one hemisphere is exposed to light while the opposite hemisphere faces away from the light. However, at any given moment, there is always one-half of the solar surface that is illuminated while the other half is in shadow.

In addition, no area of the Sun ever enters a complete dark state under normal conditions, as the Sun does not oscillate or change in shape significantly. This illumination is constant regardless of the position of the observer in relation to the Sun. Therefore, the portion of the Sun that is illuminated is essentially always half.