The correct answer is option 1: Urea is produced in the liver. Urea is a waste product that is formed in the liver during protein metabolism. When proteins are broken down in the body, one of the byproducts is ammonia, which is toxic to the body. The liver converts ammonia into a less toxic substance called urea through a series of chemical reactions known as the urea cycle. Urea is then transported to the kidneys where it is filtered out of the blood and excreted in the urine.

Option 2 is incorrect because urea is not directly produced in the blood, but rather in the liver and then transported to the blood.

Option 3 is incorrect because urea is not produced directly from the digestion of starch. Starch is broken down into glucose during digestion and then further metabolized into various substances, but urea is not one of them.

Option 4 is incorrect because urea is not produced in the lungs. It is primarily produced in the liver and processed in the kidneys.