As Isla prepares to set off fireworks, she wants to ensure the spectators are a safe distance away. When shot in the air, one firework will follow the path h(x) = -x^2 + 15x, where h(x) is the height of the firework, and x is the horizontal distance it travels in feet.

To determine how far the firework will travel before it reaches the ground, we need to solve the quadratic equation:

\[ 0 = -x^2 + 15x \]

We can start by factoring the equation. First, we can factor out \(x\) from the right side:

\[ 0 = x(-x + 15) \]

Setting each factor equal to zero gives us the possible solutions:

1. \(x = 0\)
2. \(-x + 15 = 0 \Rightarrow x = 15\)

Thus, the firework will reach the ground at the points \(x = 0\) and \(x = 15\). The significant distance we are interested in (the non-zero distance where the firework lands) is:

\[ x = 15 \text{ feet} \]

Therefore, the answer to how far the firework will travel before it reaches the ground is:

B. 15 feet