To determine the price that Regina should set to maximize her revenue, we need to find the vertex of the quadratic equation given by \( y = -0.5x^2 + 24x \).
The vertex of a parabola given by the equation \( y = ax^2 + bx + c \) can be found using the formula for the x-coordinate of the vertex:
\[ x = -\frac{b}{2a} \]
In this case, \( a = -0.5 \) and \( b = 24 \).
Substituting these values into the formula:
\[ x = -\frac{24}{2 \times -0.5} = -\frac{24}{-1} = 24 \]
Thus, the price at which Regina should sell her book to maximize her revenue is $24.
Therefore, the correct response is:
$24