**Core Concept:**
The correct answer relates to immunoglobulins, also known as antibodies, which are proteins produced by the immune system to identify and neutralize foreign substances such as bacteria, viruses, and toxins. In the context of the gastrointestinal tract, the major immunoglobulin secreted is Immunoglobulin A (IgA).

**Why the Correct Answer is Right:**
Immunoglobulin A (IgA) is the major immunoglobulin secreted by the intestinal mucosa due to its crucial role in protecting the gastrointestinal tract from harmful pathogens and antigens. IgA acts as a first line of defense against invading pathogens by neutralizing them, blocking their adherence to intestinal cells, and recruiting help from other immune cells.

**Why Each Wrong Option is Incorrect:**
A. IgA is the correct answer and not IgM (Immunoglobulin M), which is primarily involved in the early stages of the immune response.
B. IgD (Immunoglobulin D) is a less common immunoglobulin and not the major immunoglobulin secreted in the intestine.
C. IgE (Immunoglobulin E) is involved in allergic reactions and not the primary immunoglobulin protecting the intestine.
D. IgE is secreted by mast cells and basophils, not intestinal cells.

**Why Each Wrong Option is Incorrect:**
1. IgM (Option A) is involved in the early stages of the immune response and is not the primary immunoglobulin secreted in the intestine.
2. IgD (Option B) is a less common immunoglobulin and not the major immunoglobulin secreted in the intestine.
3. IgE (Option C) is involved in allergic reactions and not the primary immunoglobulin protecting the intestine.
4. IgE (Option D) is secreted by mast cells and basophils, not intestinal cells.

**Clinical Pearl:**
Understanding the immune system's role in protecting the gastrointestinal tract is crucial for practicing physicians. Intestinal immunoglobulins, particularly IgA, play a vital role in preventing pathogens from colonizing the intestinal mucosa and maintaining intestinal mucosal health.