Answer
c
Work Step by Step
Isobars represent the sum of Z (protons) and N (neutrons) inside of an atom. Due to the graph being labeled horizontally with Z and Vertically with N, a series sums equal to the same number must increase on the x and decrease on y, or increase on y and decrease on x to keep a stable value of A. This exists as a line pointed down and to the right, or up and to the left, also known as a line with a negative slope.
