The moon cycle has several phases. The moon starts off as a new moon, then waxes (or grows) into a waxing crescent moon, then it gets to its first quarter (halfway to full moon). After that it grows to its waxing gibbous stage which is more than half way illuminated. After reaching the full moon stage (fully illuminated), the moon starts to wane. The full moon becomes a waning gibbous, then reaches its third quarter stage (halfway from full moon to new moon). After waning for some more time, the moon reaches its waning crescent stage and it is almost a new moon. Upon becoming a new moon, the cycle starts again.
This whole cycle takes about 29.5 days. The full moon happens at about the halfway point (roughly 14-15 days into the cycle). This means that the moon grows for about two weeks and wanes for about 2 weeks as well. Hence, the correct answer is D. Waning (since it spends the last two weeks of the cycle waning or shrinking).
