Answer: Section C
This portion of the graph is completely flat, and has a slope of 0. The slope is the rate of change. Any horizontal line has a slope of 0 (we can use the slope formula to prove this). The term 'constant" means "it does not change".
In contrast, sections A, B, and D are all increasing sections because they go uphill as we move from left to right. The slopes of each line segment are positive.