## Core Concept
The question pertains to the structure and function of the **intestinal wall**, specifically focusing on the layers of the intestinal wall and their cellular activity in protein synthesis. The intestinal wall is primarily composed of four layers: mucosa, submucosa, muscularis, and serosa.

## Why the Correct Answer is Right
The **mucosa** is the innermost layer of the intestinal wall, directly involved in the absorption of nutrients. It contains **enterocytes**, which are specialized cells responsible for nutrient absorption. These cells are highly active in protein synthesis to facilitate the production of enzymes and transport proteins necessary for nutrient uptake and processing. The high metabolic activity and the need for continuous renewal of the mucosal lining make the cells in this layer particularly active in protein synthesis.

## Why Each Wrong Option is Incorrect
**Option A:** The **serosa** is the outermost layer of the intestinal wall, providing a smooth surface for organs to slide against. It is primarily composed of connective tissue and does not require high levels of protein synthesis for its primary functions.

**Option B:** The **muscularis** layer consists of smooth muscle cells that are responsible for the movement of the intestine (peristalsis). While these cells do engage in protein synthesis for muscle contraction and relaxation, their activity is not as high as that of cells involved in absorption and secretion.

**Option C:** The **submucosa** is a layer of loose connective tissue beneath the mucosa. It supports the mucosa and contains blood vessels and nerves. Although it does have cells that engage in protein synthesis, the level of activity is not as high as in the mucosa.

## Clinical Pearl / High-Yield Fact
A key point to remember is that the **mucosa**, being the layer directly involved in nutrient absorption, has a high turnover rate and is richly supplied with blood and lymphatic vessels. This layer's high metabolic activity makes its cells particularly active in protein synthesis to support rapid cell turnover and functional demands.

## Correct Answer: D. Mucosa.