In a right triangle, the sides \( a \) and \( b \) are the legs, and \( c \) is the hypotenuse. For the Pythagorean triple (9, 12, 15), \( c \) is the hypotenuse, which means it is the longest side. Thus, \( c = 15 \) and the legs \( a \) and \( b \) can be either 9 or 12.
Therefore, the correct value of \( a \) can be either 9 or 12. Based on the provided options, the possible value of \( a \) is:
- \( a = 9 \)
- \( a = 12 \)
Since only one of the answers can be chosen, and since both 9 and 12 are valid lengths for the legs, the correct answer to select from the given options is:
a = 9