Ammonia is the end product of 

The correct answer is C.

As a byproduct of protein metabolism, ammonia is constantly released (transamination and deamination). This ammonia is poisonous to the body. As a result, it is detoxified in the animals' livers. The ornithine cycle detoxifies ammonia to create urea. Because urea is less poisonous than ammonia, it can stay inside the body for a longer amount of time without causing any harm.