**Core Concept**
The fetus's immune system is still developing, and it relies on the mother's antibodies for protection during the early stages of life. However, once the fetus's immune system matures, it starts producing its own antibodies, primarily through the secretion of immunoglobulins. This process is crucial for the development of the fetus's immune system and its ability to respond to infections.

**Why the Correct Answer is Right**
The primary immunoglobulin secreted by the fetus as a primary response is IgG. This is because IgG is the most abundant antibody isotype in the fetus's circulation, and it is produced by the fetus's immune system in response to infections and other antigens. IgG is also the only antibody isotype that can cross the placenta from the mother to the fetus, providing passive immunity during the early stages of life. However, as the fetus's immune system matures, it starts producing its own IgG antibodies, which are essential for the development of active immunity.

**Why Each Wrong Option is Incorrect**
**Option A:** IgM is the first antibody isotype to be produced in response to an infection, but it is not the primary immunoglobulin secreted by the fetus.
**Option B:** IgA is primarily produced in the mucosal surfaces and is not the primary immunoglobulin secreted by the fetus.
**Option C:** IgE is involved in allergic reactions and is not the primary immunoglobulin secreted by the fetus.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the fetus's immune system is still developing, and it relies on the mother's antibodies for protection during the early stages of life. However, once the fetus's immune system matures, it starts producing its own antibodies, primarily through the secretion of IgG.

**Correct Answer:** A. IgG.