**Core Concept**
The transfer of antibodies from mother to fetus is a crucial aspect of fetal immunity, providing protection against infections during the neonatal period. This process is mediated by the placenta, which allows specific antibody isotypes to cross the maternal-fetal barrier.

**Why the Correct Answer is Right**
IgG is the primary antibody isotype transferred from mother to fetus, accounting for approximately 95% of the immunoglobulins present in cord blood. This is due to the relatively small size of IgG molecules, which allows them to pass through the placenta's tight junctions. The placental transfer of IgG is facilitated by the FcRn receptor, which binds to the Fc region of IgG and protects it from degradation in the fetus's circulation. This ensures that the fetus receives maternal IgG antibodies, providing passive immunity against infections such as tetanus, hepatitis B, and rubella.

**Why Each Wrong Option is Incorrect**
**Option B:** IgM is the first antibody isotype produced in response to an infection, but it is too large to cross the placenta, making it an ineffective mediator of maternal-fetal antibody transfer.
**Option C:** IgD is primarily present on mature B cells and plays a role in activating these cells, but it is not involved in maternal-fetal antibody transfer.
**Option D:** IgA is the primary antibody isotype found in mucosal secretions and is involved in mucosal immunity, but it does not cross the placenta in significant amounts.

**Clinical Pearl / High-Yield Fact**
The transfer of IgG antibodies from mother to fetus is a critical component of fetal immunity, and the levels of these antibodies in cord blood can serve as an indicator of maternal immunity status. It is essential to ensure that pregnant women are immunized against diseases such as tetanus and hepatitis B to provide optimal protection for their newborns.

**✓ Correct Answer: A. IgG**