What is the median of the following set of numbers: 3, 7, 9, 12, 15?

To find the median of a set of numbers, you first need to arrange the numbers in ascending order, if they are not already. In this case, the numbers are already in order: 3, 7, 9, 12, 15.

The median is the middle number in a sorted list. Since there are five numbers in this set (an odd quantity), the median will be the third number. Counting to the third number in the sorted list, we have:

1. 3

2. 7

3. 9 (this is the median)

4. 12

5. 15

Therefore, the median of this set of numbers is 9, which is why this answer is the correct choice.