The number of registered voters in a district is modeled by a decreasing exponential function,...

A decreasing exponential function falls quickly at first and then levels off over time. The “slowest average decrease” occurs where the graph is flattest, because the output changes by the smallest amount per year. Earlier intervals, such as 2005 to 2008, occur when the graph is steepest, so those intervals have a faster average decrease. Later intervals occur after the exponential decay has slowed. From the graph, the interval from 2015 to 2025 is farthest to the right and has the least steep downward trend. Therefore, the number of registered voters decreases most slowly during that interval. This is why the correct answer is D.