Explanation:
The correct answer is D: liver.

Excess amino acids in the body are broken down in the liver through a process called deamination. During this process, the amino group from the amino acids is removed and converted into ammonia, which is toxic. The ammonia is then converted into urea through the urea cycle. This urea is less toxic and can be safely excreted from the body through the kidneys.

Here's why the other options are incorrect:
- A. Kidney: While the kidneys are responsible for filtering waste products from the blood, including urea, the conversion of excess amino acids into urea occurs in the liver, not the kidneys.
- B. Pancreas: The pancreas is involved in producing digestive enzymes and insulin, but not in the conversion of amino acids to urea.
- C. Villi: The villi are structures in the small intestine that absorb nutrients, not involved in the conversion of amino acids.
- E. Spleen: The spleen helps filter blood and recycle iron from red blood cells, but it is not involved in amino acid conversion.

Thus, D: liver is the correct answer.