**Core Concept:**
The septum transversum is a transient embryonic structure that forms part of the diaphragm during development. The diaphragm is a crucial muscle responsible for respiratory function, separating the thoracic cavity from the abdominal cavity. It consists of two layers: the pleural layer (parenchymal layer) and the pericardial layer (muscular layer).

**Why the Correct Answer is Right:**
The correct answer is **D.** The septum transversum contributes to the formation of the pleural layer (parenchymal layer) of the diaphragm. During embryonic development, the pleural layer originates from the mesoderm tissue found on the ventral side of the septum transversum. This layer contains thin, flattened cells called pleura, which are responsible for lining the thoracic cavity.

**Why Each Wrong Option is Incorrect:**
A. The septum transversum does not give rise to the pericardial layer (muscular layer) of the diaphragm. Instead, the pericardial layer arises from the somites, which are embryonic segments of the paraxial mesoderm.

B. The pleural layer and pericardial layer are distinct parts of the diaphragm, and the septum transversum contributes to the formation of only the pleural layer.

C. The pleural layer forms from the pleural mesoderm, which is separate from the mesoderm involved in forming the septum transversum.

**Clinical Pearl:**
A better understanding of embryonic development can help medical students and physicians appreciate the complexity and interconnectedness of organ systems. In this case, it highlights the importance of knowing the differentiation pathways of different layers of the diaphragm to comprehend the formation of this vital respiratory organ.