Final answer:
Susan's unit rate per gallon of gas is 40 miles per gallon, which is found by dividing 120 miles by 3 gallons. The option (4) is correct.
Step-by-step explanation:
To calculate Susan's unit rate per gallon of gas, divide the total miles driven by the total gallons of gas used. The question asks for the unit rate per gallon, not total miles or total gallons, and not the unit rate per mile.
Therefore, we divide 120 miles by 3 gallons, which gives us 40 miles per gallon as Susan's unit rate of gas consumption. This unit rate allows us to understand how efficient a vehicle is with fuel consumption. Therefore, option (4) is correct.