Final answer:
Water transport is the least expensive mode among the given options, with ships being more efficient and capable of carrying a significant amount of cargo.
Step-by-step explanation:
The least expensive mode of transportation among the options given is water transport.
According to Table D.2, ships are about four times more efficient per ton compared to trucks and can carry 5,000 times more cargo.
Additionally, air travel can be substantially more energy-intensive due to the large distances involved.