Constant means unchanging. You can see that there is a portion in the graph where the line is just flat. That means that the graph is constant in that interval. Find where the constant line starts and ends. Since the coordinates are increasing by 2, and the constant line starts between 0 and the first tick (2), the constant line starts at 1. Do the same thing to find where it ends(the last point where the line is flat). This should be 6.
That means the line is unchanging(constant) from 1 to 6, which can be represented by C.
