Final answer:
The term 'wane' describes the moon's phase where it decreases in illumination after a full moon, opposing the 'waxing' phase.
Step-by-step explanation:
The correct answer to the question is a) Wane. This term refers to the moon's phases when it decreases in strength or visibility in the night sky, following the full moon phase. Waning is indeed the opposite of waxing, which describes the period when the moon's illuminated side is increasing. After the full moon, it transitions to the waning gibbous, then to the last quarter, the waning crescent, and finally back to the new moon. This lunar cycle is approximately 29.5 days long, and it has been observed since ancient times to track time. The phenomenon of the moon 'waxing' and 'waning' provides the origins of words such as 'month' and 'Monday'.