To solve the problem, we first need to convert the angle of the hill from degrees to radians. The angle given is \(15^\circ\). The conversion from degrees to radians is done using the formula:
\[
\text{angle in radians} = \text{angle in degrees} \times \frac{\pi}{180}
\]
Substituting the given angle:
\[
\text{angle in radians} = 15 \times \frac{\pi}{180} \approx 0.2618
\]
We use the tangent function to relate the vertical gain and the horizontal distance. The tangent of an angle in a right triangle is the ratio of the opposite side (vertical gain) to the adjacent side (horizontal distance). Therefore, we have:
\[
\tan(\theta) = \frac{\text{vertical gain}}{\text{horizontal distance}}
\]
Rearranging for horizontal distance:
\[
\text{horizontal distance} = \frac{\text{vertical gain}}{\tan(\theta)}
\]
Substituting the known values:
\[
\text{horizontal distance} = \frac{259}{\tan(0.2618)} \approx 966.6012
\]
Finally, we round the calculated horizontal distance to the nearest foot:
\[
\text{horizontal distance rounded} = 967
\]