Question

Approximately what percent of the Moon’s surface is illuminated on a new-moon day? A) 0% B) 20% C) 50% D) 100%

Answer 1

50% (half) of the moon's surface is illuminated at all times. At the time of New Moon, because of the moon's position between us and the sun, the illuminated half is turned completely away from us, and we can't see any of it.

Answer 2

At New Moon the percent illuminated is 0; at First and Last Quarters it is 50%; and at Full Moon it is 100%. During the crescent phases the percent illuminated is between 0 and 50% and during gibbous phases it is between 50% and 100%.
so it depends on the phase