The statement is incomplete for Model C, as the maximum depth of water is not provided. Therefore, we cannot determine which of the three pools needs the smallest amount of water to be completely filled.
If we assume that the maximum depth of water for Model C is 4 feet (which is the same as Model A), then we can compare the volumes of the three pools:
The volume of Model A is 18 ft x x ft x 4 ft = 72x cubic feet
The volume of Model B is 16 ft x 10 ft x 5 ft = 800 cubic feet
The volume of Model C is 15 ft x 12 ft x 4 ft = 720 cubic feet
From this calculation, we can see that Model C needs the smallest amount of water to be completely filled, as its volume is 720 cubic feet, which is less than the volumes of Models A and B. Therefore, the answer is Model C.
However, it's important to note that this answer is based on the assumption that the maximum depth of water for Model C is 4 feet. If the actual maximum depth of water is different, the answer may be different as well.