Answer:
The correct option is 21
Explanation:
First of all, it should be noted that the full moon is usually observed in the middle of the lunar month, which is usually 29 or 30 days, hence the full moon can be observed either on the 13th but most likely on the 14th and/or the 15th of the lunar month.
At the early days of the month, the new moon begins to form a crescent which appears and continues to get fuller (called waxing crescent) as the day goes by until half of the moon is noticed (usually after about seven days) on one side, this is known as the first quarter.
After half of the moon is noticed on one side, the moon continues to get fuller (called gibbous moon) until a full moon is noticed on the 14th/15th day. <u>This full moon then starts waning or becoming smaller until the other half of the moon becomes visible while the initially seen half becomes invisible/clouded (this is called </u><u>third quarter</u><u>)</u>. This happens after another 7 days or on the 21st day. This half moon continues to get smaller (forming a waning crescent) until no moon is seen at the end of the month (around the 28 or 29th day) which is referred to the new moon.
Hence, we can see from the above that from the third quarter to a new full moon will go through the steps below
third quarter ⇒ new moon ⇒ first quarter ⇒ full moon
If each step takes about 7 days and there are 3 steps, the approximate number of days she has to wait before the full moon is 21 days.
