1. The problem is to find the median of the data set: 11, 4, 6, 15, 21, 19, 0, 19, 25. 2. The median is the middle value when the data is arranged in ascending order. 3. First, arrange the numbers in ascending order: 0, 4, 6, 11, 15, 19, 19, 21, 25. 4. Since there are 9 numbers (an odd number), the median is the value at position $\frac{9+1}{2} = 5$. 5. The 5th number in the ordered list is 15. 6. Therefore, the median of the data set is $15$.