**Core Concept**
The question is testing the student's knowledge of nitrogen metabolism in the human body. Nitrogen is a crucial component of amino acids, which are the building blocks of proteins. The body's nitrogen balance is essential for maintaining protein homeostasis.

**Why the Correct Answer is Right**
The correct answer is that the nitrogen of the body is supplied by the breakdown of proteins, primarily through the process of deamination. Deamination is the removal of an amino group (-NH2) from an amino acid, resulting in the production of ammonia (NH3) and a keto acid. This process occurs in various tissues, including the liver, kidneys, and muscles. The ammonia produced is then converted into urea in the liver, which is excreted through the urine. This process maintains nitrogen balance in the body.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not specify a source of nitrogen. While the body does recycle nitrogen from amino acids, it is not the primary source of nitrogen.
**Option B:** This option is incorrect because it mentions the gut, which is not a primary site of nitrogen metabolism in the body. While the gut does absorb nitrogen from dietary proteins, it is not the primary source of nitrogen.
**Option C:** This option is incorrect because it mentions the kidneys, which are involved in the excretion of nitrogen waste products, but not the primary source of nitrogen.

**Clinical Pearl / High-Yield Fact**
The body's nitrogen balance is tightly regulated, and an imbalance can lead to various clinical conditions, including uremia and hepatic encephalopathy. Understanding the process of deamination and nitrogen metabolism is crucial for managing patients with liver disease or renal failure.

**Correct Answer:** C.