What is the most important way to save energy and money in transportation? 

A)  increase the fuel efficiency of motor vehicles 
B)  switch to hydrogen-powered cars 
C)  switch to electric engines 
D)  ban cars in cities

Increasing the fuel efficiency of motor vehicles is the most important way to save energy and money in transportation because it can be done immediately and does not require a complete overhaul of the current transportation infrastructure. This can be achieved through measures like using lighter materials, improving aerodynamics, and using more efficient engines. Switching to hydrogen-powered cars or electric engines may be beneficial in the long run, but these technologies are still in development and require significant infrastructure investment. Banning cars in cities is not a practical solution as many people rely on cars for transportation and there are other ways to reduce emissions and improve air quality in urban areas.