The correct answer is - 500.
The contour lines on the presented picture are at an elevation distance of 500 meters. The way that we can conclude that is that the top of the mountains is not marked with a contour line itself, as in pretty much all cases it is above the last contour line, and bellow the contour line that would come next in order, so in between height.
In this topographic map we have 5 contour lines, so 5 x 500 is 2,500 m of elevation. The next contour line would come on 3,000 m, but Mt. St. Helens doesn't have that elevation as it is lacking 50 m, so the last contour line is the one on 2,500 m, while the top of the mountain will be marked with a special sign.