Answer:
Gallons per minute, because the independent quantity is volume of water in gallons and dependent quantity is time in minutes.
Step-by-step explanation:
As we know that gallons is the unit of volume which is used in US to measure volume of gas as well as liquids while time is measured in minutes.
Now we know that if we need to find the volume flow rate then in that case the measurement is done to find the amount of fluid flow per unit of time
so here since volume flow is an independent variable which do not depends on any other parameter while time is a dependent variable here which is given as

so correct answer will be
Gallons per minute, because the independent quantity is volume of water in gallons and dependent quantity is time in minutes.