Final answer:
An increase in subsidies for fuel-efficient cars to automakers would shift the supply of fuel-efficient cars to the right.
Step-by-step explanation:
The event that would shift the supply of fuel-efficient cars to the right is an increase in subsidies for fuel efficient cars to automakers. When there is an increase in subsidies, it reduces the cost for automakers to produce fuel-efficient cars and incentivizes them to produce more. This increases the supply of fuel-efficient cars in the market.