The correct answer is C) 125 feet.
The other options of the question were A) 25 feet. B) 100. C) 175 feet.
The difference in elevation between index contours is 125 feet.
Here, we are talking about the fixed distance of two contour lines on a topography map. This is a detailed map of the surface of the planet, used mostly in urban planning when engineers are about to study the land surface before initiating a project as part of the feasibility studies.