**Core Concept:** The first external sign of decomposition is mainly related to the process of putrefaction, which is an early stage of decomposition following death. This process involves the breakdown of organic compounds due to microbial activity, leading to visible changes on the body's surface.

**Why the Correct Answer is Right:** The correct answer, **D. Discoloration (Gangrene)**, represents the initial visible alteration that occurs during decomposition. Discoloration, also known as gangrene, is characterized by the discoloration of the skin due to the accumulation of gases, particularly carbon dioxide, in the tissue. This gas formation is a result of anaerobic bacterial fermentation, which occurs when oxygen is depleted in the tissues.

**Why Each Wrong Option is Incorrect:**

A. **Tautness (Tautness)** is not a correct answer as it doesn't represent an external sign of decomposition. After death, the body loses tautness due to the cessation of muscle contractions caused by the loss of the neuromuscular connection between the brain and muscles.

B. **Froth (Exhalation)** is not a correct answer since froth is more associated with respiratory issues and not a characteristic of decomposition. During decomposition, gases are produced within the tissues, which results in discoloration and gangrene.

C. **Belly button (Navel)** is not a correct answer as it doesn't represent an external sign of decomposition. While some changes might occur at the navel due to decomposition, it is not a universally applicable sign across all decomposing bodies.

**Clinical Pearl:** A proper understanding of decomposition signs is crucial in forensic medicine as well as in clinical practice when dealing with patients experiencing tissue necrosis or other conditions leading to tissue breakdown.

**Explanation:**

In the context of decomposition, D. Discoloration (Gangrene) is the correct answer because it is the first visible alteration that occurs on the body surface due to gas accumulation in the tissues caused by anaerobic bacterial fermentation. This process is initiated when the body's oxygen levels decrease, allowing anaerobic bacteria to thrive and produce gases like carbon dioxide, hydrogen, and methane, which lead to discoloration and gangrene.

The other options are incorrect because they either represent physiological changes unrelated to decomposition (A: Tautness; B: Froth; C: Belly button), or they are not specific enough to be universally applicable signs of decomposition (A, B, and C). In forensic medicine and clinical practice, understanding these signs helps in assessing the post-mortem interval (the time since death) and in identifying the cause of death in cases involving decomposition or tissue necrosis.