Final answer:
The organelle that is not properly functioning in the fish cell is the mitochondria. Therefore, the correct option is B.
Step-by-step explanation:
The organelle in the fish cell that is not properly functioning is the mitochondria.
Mitochondria are responsible for producing ATP through cellular respiration, using energy from organic compounds. If the ATP levels are decreasing rapidly, it suggests that the mitochondria may not be functioning properly, leading to a decrease in ATP production.
For example, if the inner membrane of the mitochondria is damaged or dysfunctional, it can affect the electron transport chain and prevent the proper generation of ATP. This can ultimately lead to a decrease in energy for the cell and contribute to the observed decrease in ATP levels.