The circumference of the circle is given by the following formula:
2 * π * radius
Since the diameter equal two times the radius, that can also be written as:
2 * π * radius = diameter * π
Therefore, comparing the above formula with the given value of the circumference, we see that:
diameter * π = 15π in
Dividing both sides of this equation by π, we find:
diameter * π/π = 15π in/π
diameter = 15 in
Thus, the last option is correct.