ANSWER
A. Flat (not steep)
Step-by-step explanation
If the graph is a vertical line, then the slope is undefined.
If the graph is a horizontal line, then the slope is zero.
Any of these is the case of this graph, which leaves us with the other two classifications. Because the option "flat (not steep)" is a different option than zero, this has to be the classification for a slope that is less than 1, while a larger slope is classified as steep.
Hence, the answer is option A. Flat (not steep).