**Core Concept:** The process of putrefaction is the decomposition of organic matter, particularly of dead organic tissues, by microorganisms like bacteria and fungi. In the human body, putrefaction starts with soft tissues and progresses to bones, organs, and finally the brain.

**Why the Correct Answer is Right:** The correct answer, **D. Brain**, is the last organ to putrefy because it is the last to become devoid of oxygen and nutrients due to the process of rigor mortis, during which blood flow to the limbs decreases and the heart contracts uncontrollably. As a result, the brain is the last to receive sufficient nutrients and oxygen to support microbial growth and putrefaction.

**Why Each Wrong Option is Incorrect:**

A. **Option A (Skin)** is incorrect because the skin, being the outermost layer, is the first to be exposed to environmental factors and microorganisms, making it the first to undergo putrefaction.

B. **Option B (Muscle)** is incorrect because, as mentioned above, rigor mortis leads to decreased blood flow to muscles, making them a less hospitable environment for microbial growth compared to the brain, which still receives some blood flow.

C. **Option C (Organs)** is incorrect because organs are generally more protected and have less direct exposure to the external environment compared to the skin and muscles. However, they still undergo putrefaction, making this option less specific than the correct answer.

**Clinical Pearl:** The process of putrefaction is a critical aspect of forensic medicine, where it helps in determining the time of death (post-mortem interval) by observing the stages of putrefaction in different body parts.

**Correct Answer:** **D. Brain** is the last organ to putrefy due to the described reasons above.