That would be mass.
Hope it helps!
Given:
The solution decreased by
ounces every hour for 5 hours.
Remaining solution at the end of the experiment =
ounces.
To find:
The initial amount of solution.
Solution:
Let the initial amount of the solution be x.
The solution decreased by
ounces every hour for 5 hours.
Total decreased amount 

Initial amount = Remining amount + Decreased amount of solution.




Therefore, the initial amount of solution is 18 ounces.
I believe the answer is A!
Do you have the solutions you want me to compare and contrast? :)