Answer: A. A relation that is a function.
Explanation:
A relation is said to be a function if each input value is associated with one unique output value.
Here, Number of miles run by car is directly proportional to the quantity of gasoline.
Independent variable - quantity of gasoline.
Dependent variable - Number of miles
On assuming the condition of car as good, the number of miles a car can run highly depends on the gallons of gasoline in the tank .
As quantity of gasoline increases number of miles driven would also increases.
i.e. Number of miles driven is a function with respect to quantity of gasoline.
Hence, the given line best describe as : A relation that is afunction.