**Core Concept**
The process of decomposition in a dead body is a complex sequence of biochemical and microbiological reactions. It involves the breakdown of tissues, the putrefaction and decomposition of the body, and the eventual return of nutrients to the environment. The first change in a dead body during decomposition is a critical indicator of the onset of this process.

**Why the Correct Answer is Right**
The first change in a dead body during decomposition is the loss of rigidity, also known as algor mortis. This occurs due to the cooling of the body after death, which leads to the cessation of metabolic processes and the breakdown of cellular structures. As the body cools, the muscles relax, and the rigid post-mortem stance is lost. This change is often noticeable within 2-4 hours after death.

**Why Each Wrong Option is Incorrect**
**Option A:** Putrefaction is the process of decomposition caused by the action of bacteria and other microorganisms. While it is an important aspect of decomposition, it is not the first change that occurs in a dead body.

**Option B:** Livor mortis, also known as post-mortem lividity, is the pooling of blood in the lower extremities due to gravity. It is a common finding in dead bodies but occurs after algor mortis.

**Option C:** Marbling refers to the appearance of veins and arteries visible under the skin due to the separation of blood from the tissues. It is not the first change that occurs in a dead body during decomposition.

**Clinical Pearl / High-Yield Fact**
The first change in a dead body during decomposition, algor mortis, is a critical indicator of the post-mortem interval (PMI). The rate of cooling of the body can provide valuable information about the time elapsed since death.

**Correct Answer: C. Marbling is the appearance of veins and arteries visible under the skin due to the separation of blood from the tissues.