Final answer:
The closer the contour lines are on a map, the steeper the gradient of the slope is. This is analogous to how electric fields strengthen with closer isolines of electric potential.
Step-by-step explanation:
The missing word that completes the sentence 'The closer the contour lines are, the _______ the gradient of the slope' is steeper. This concept is similar across various fields, including geography and physics. For instance, as isolines of electric potential get closer together, the electric field gets stronger. Analogously, if we consider the isolines of gravitational potential representing the shape of a hill, they reveal that the slope of the hill is steeper where these isolines are closer together. If the slope of a line increases, graphically, it becomes steeper - meaning that it has a steeper upward or downward tilt, depending on whether the slope is positive or negative.
Therefore, the sentence correctly filled would be: 'The closer the contour lines are, the steeper the gradient of the slope.'