**Core Concept**
The question is testing the student's knowledge of mucosal immunity, particularly the role of immunoglobulins in protecting against infections in the respiratory and gastrointestinal tracts.

**Why the Correct Answer is Right**
Immunoglobulin A (IgA) is the most abundant immunoglobulin at mucosal surfaces, including the respiratory and gastrointestinal tracts. It plays a crucial role in neutralizing pathogens and toxins, thereby preventing their entry into the body. IgA is secreted in large quantities into mucosal secretions such as saliva, tears, and mucus, where it provides a first line of defense against infections.

**Why Each Wrong Option is Incorrect**
* **Option A:** IgM is the first immunoglobulin to be produced in response to an infection and is primarily involved in the ABO blood group antigen response. It is not the most affected immunoglobulin in respiratory and GIT infections.
* **Option B:** IgG is the most abundant immunoglobulin in the blood and provides long-term immunity against infections. While it is involved in the immune response, it is not the most affected immunoglobulin in respiratory and GIT infections.
* **Option D:** IgE is involved in allergic reactions and is not the most affected immunoglobulin in respiratory and GIT infections.

**Clinical Pearl / High-Yield Fact**
The gut-associated lymphoid tissue (GALT) is the largest component of the immune system, with a high concentration of IgA-producing plasma cells. This highlights the importance of IgA in mucosal immunity and its role in protecting against infections in the respiratory and gastrointestinal tracts.

**Correct Answer: C. Immunoglobulin A.