For bowed-out PPFs, the opportunity cost of producing scooters is reflected in the curvature of the PPF. In flatter regions, producing an additional bike requires giving up fewer scooters. However, in steeper regions, producing an additional bike requires giving up more scooters. In other words, the opportunity cost of producing bikes changes as you move along the PPF.
For linear PPFs, the opportunity cost of producing bikes is constant and reflected in the slope of the PPF. If the PPF is flatter, producing an additional bike requires giving up fewer scooters. If the PPF is steeper, producing an additional bike requires giving up more scooters.
In this case, because the opportunity cost of producing additional bikes remains constant as more resources are shifted to the production of bikes, the PPF must be linear. Therefore, Graph 1 best represents the trade-off Scotland faces between producing bikes and scooters.