Final answer:
Walking as a form of transportation is limited by its narrow range of optimal speed, making it less effective over long distances and less adaptable to time constraints.
Step-by-step explanation:
The downside of walking as a way of transportation is (d) has a narrow range of optimal speed. Walking is slower compared to other modes of transportation like biking or driving. It is suitable for short distances but becomes time-consuming over longer distances. Another point to consider is that walking limits the speed one can achieve, as compared to a vehicle or even bicycling, and does not allow for quick adjustment to sudden time constraints or emergencies.