**Core Concept**
Postmortem changes in the body occur due to a combination of autolysis, putrefaction, and decomposition. Marbling refers to the appearance of a network of blood vessels on the skin's surface, usually visible as a result of the breakdown of blood components and the diffusion of red blood cells into the surrounding tissues.

**Why the Correct Answer is Right**
Marbling typically becomes visible within 24 to 72 hours after death, as the process of putrefaction accelerates and the breakdown of blood components leads to the appearance of a characteristic network of blood vessels on the skin's surface. This is due to the action of various enzymes, including lipases, proteases, and phospholipases, which break down the blood components and lead to the diffusion of red blood cells into the surrounding tissues. The visibility of marbling is influenced by factors such as the ambient temperature, the presence of bacteria, and the condition of the body at the time of death.

**Why Each Wrong Option is Incorrect**
**Option A:** 6-12 hours - This is too early for marbling to become visible, as the process of putrefaction and decomposition is still in its initial stages.
**Option B:** 1-2 weeks - This is too late for marbling to become visible, as the body would have undergone significant decomposition and the skin would have begun to slough off.
**Option C:** 3-5 days - While marbling may be visible by this time, it is not the most accurate estimate, as the process can vary significantly depending on the factors mentioned earlier.

**Clinical Pearl / High-Yield Fact**
It is essential to note that the appearance of marbling can be influenced by various factors, including the ambient temperature, the presence of bacteria, and the condition of the body at the time of death. Therefore, a more accurate estimate of the time required for marbling to appear can only be made by considering these factors on a case-by-case basis.

**Correct Answer:** C. 3-5 days.