The problem says the right size for 3 miniature goldfish is a 9-gallon aquarium, then the ratio is:

If we divide the numerator and the denominator by 3, we will obtain:

Thus, we need 3-gallon for each 1 miniature goldfish, then in the options we can see two that apply:

Then (6,18) is one of the options
The other one is (1,3) because it fits the ratio.
Answer: option A. (6,18) and option D. (1,3)