Explanation:
> "Why does a flamingo stand on one leg? Because if it lifted the other one, it would ____."

The sentence is explaining a cause and effect situation where the flamingo stands on one leg to avoid a negative outcome. The most logical negative outcome would be the flamingo losing balance and falling.

Now, let's look at the options:

- A. go down — This is not the most appropriate choice in this context, as "go down" is usually used for movement, not falling due to imbalance. Incorrect.
- B. let down — "Let down" typically refers to disappointing someone or lowering something, not falling. Incorrect.
- C. fall down — This is the most natural choice. It directly explains the consequence of lifting the other leg — the flamingo would lose balance and fall. Correct.
- D. come down — This would typically refer to descending, not losing balance or falling. Incorrect.
- E. get down — "Get down" usually refers to moving downward but doesn’t specifically fit the idea of falling due to imbalance. Incorrect.