Final answer:
Fossil fuels are the primary source of energy for transportation, although there is a growing interest in renewables like solar power, wind energy, and hydropower.
Step-by-step explanation:
The world primarily uses fossil fuels such as oil, coal, and natural gas for transportation energy. These non-renewable energy sources are finite and contribute to global warming and climate change due to the carbon dioxide released when they are burned.
Nuclear energy, while a significant source of electricity, is not widely used for transportation. Instead, it provides low carbon electricity but entails challenges with storing spent fuel. Renewables like solar power, wind energy, and hydropower are being explored as alternatives, although currently, they are more suited to electricity generation rather than direct use in transportation due to issues with storage and energy density.