Final answer:
The moon is not illuminated 50% during phases other than the first quarter and third quarter, i.e., any time it is not a half moon.
Step-by-step explanation:
The moon is not illuminated 50% during any phase that is not a half moon, which means the correct answer to the student's question is b) any time it is not a half moon. The first quarter and third quarter phases are the ones where half of the Moon's visible face is illuminated, which occurs about one week after the new moon and one week after the full moon, respectively. The times when it will not be illuminated at 50% are during the waxing crescent, waxing gibbous, waning gibbous, and waning crescent phases. A lunar eclipse does not strictly correspond to the amount of illumination, although during a total lunar eclipse the Moon is in Earth's shadow and thus not illuminated by direct sunlight.