The most common and usually cheapest option is "full-to-full": you receive the car with a full tank and are expected to return it full, paying only for the fuel you personally use at local pump prices. This works best if you're comfortable finding a gas station near the return location before dropping off the car.
Prepaid fuel means paying for a full tank upfront, regardless of how much you actually use, and returning the car empty or partially empty without penalty. It's convenient if your schedule is tight at drop-off, but it usually costs more overall unless you're confident you'll use nearly the entire tank.
The option to actively avoid is "return without refueling and pay the rental company's rate" — being charged for missing fuel at the counter almost always costs significantly more per gallon or liter than filling up yourself, sometimes double the local pump price or more.
