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To find the missing number in the group of four, we start by understanding the concept of an average. The average is calculated by adding all the numbers together and then dividing by the total count of numbers. In this case, the average of four numbers is given as 12. Therefore, to find the total sum of these four numbers, we multiply the average by the total count of numbers:

\[ \text{Total Sum} = \text{Average} \times \text{Number of Values} = 12 \times 4 = 48. \]

Next, we need to sum the known numbers, which are 9, 12, and 15:

\[ 9 + 12 + 15 = 36. \]

To find the missing number, we subtract the sum of the known numbers from the total sum we calculated:

\[ \text{Missing Number} = \text{Total Sum} - \text{Sum of Known Numbers} = 48 - 36 = 12. \]

Thus, the missing number that, when combined with 9, 12, and 15, results in an average of 12 is 12. This is why the correct answer is 12; it ensures