Final answer:
Fuel efficiency is an important factor to consider when buying a vehicle, as it affects long-term fuel costs and environmental impact.
Step-by-step explanation:
When deciding what type of vehicle to buy, one important factor to consider is fuel efficiency (a). Fuel efficiency is significant because cars with better fuel efficiency get more miles per gallon, which can save money on fuel costs and is better for the environment. Considering that cars are becoming more fuel efficient, you would benefit in the long term from a vehicle that aligns with this trend. Exterior color (b), brand popularity (c), and wheel size (d) are factors that may influence personal preference but do not inherently affect the vehicle's performance or cost-effectiveness.