Answer:
the unit cost per item
Step-by-step explanation:
The description of the graph tells you what the slope means. The slope is the (change in) y-value (cost) per (change in) x-value (item), thus is cost per item.
The slope of the line from the origin to a point is the average cost for that number of items. The slope of a tangent line at a point on the curve is the marginal cost for that number of items.
When the curve is linear and goes through the origin, the slope is the same everywhere and is the cost per item. (It is also the average cost and the marginal cost, where that is of interest.)
