What is the median of the following data set: 3, 7, 9, 4, 5?

To find the median of a data set, you need to arrange the numbers in ascending order and then identify the middle value.

First, let's order the given numbers: 3, 7, 9, 4, 5. When these numbers are arranged in ascending order, they become: 3, 4, 5, 7, 9.

Next, since there are five numbers in this data set (an odd amount), the median is the middle number. The middle number in this ordered list is the third number, which is 5.

Thus, the median of the data set is 5, which aligns with the correct answer. This method of finding the median can be applied to any data set, ensuring that the values are properly ordered first.