In this context, the slope of 1.5 in the graph representing the relationship between the number of pancakes on a plate (x-axis) and the total weight of the plate of pancakes (y-axis) means that for every additional pancake added to the plate, the total weight increases by 1.5 ounces.
Thus, your interpretation is correct:
For every additional pancake, the total weight increases by 1.5 ounces.
The other interpretation you included, "There are 1.5 pancakes on the plate," is not accurate in this case, as the slope specifically refers to the change in weight (oz) per unit increase in the number of pancakes (pancakes), rather than suggesting a fractional quantity of pancakes.