To find the least and greatest number of fish in the aquarium, we can use the two extremes for the number of fish per habitat: the minimum (15 fish) and the maximum (24 fish).
Minimum Calculation:
- If each of the 32 habitats has the minimum of 15 fish, the total number of fish would be: \[ 32 \times 15 = 480 \]
Maximum Calculation:
- If each of the 32 habitats has the maximum of 24 fish, the total number of fish would be: \[ 32 \times 24 = 768 \]
Thus, the least number of fish they could have is 480 fish, and the greatest number of fish they could have is 768 fish.
Therefore, the answer is: A. 480 fish and 768 fish.