**Core Concept**
The transfer of immunoglobulins across the placenta is an essential aspect of fetal immune system development, and some IgG subclasses have unique properties that facilitate this process. The ability of an immunoglobulin to activate complement is a crucial function in the immune response, particularly in the elimination of pathogens.

**Why the Correct Answer is Right**
IgG1 is the primary immunoglobulin subclass that crosses the placenta readily due to its relatively small size and lack of Fc receptors on placental cells. This property allows IgG1 to provide essential passive immunity to the fetus, protecting against infections during the perinatal period. IgG1 can activate the classical pathway of complement activation by binding to C1q, initiating a cascade of reactions that ultimately lead to the formation of the membrane attack complex (MAC) and the destruction of pathogens.

**Why Each Wrong Option is Incorrect**
**Option A:** IgG4 is an IgG subclass that does not cross the placenta as readily as IgG1 due to its larger size and higher affinity for Fc receptors on placental cells.
**Option B:** IgA is primarily found in mucosal secretions and does not cross the placenta in significant amounts.
**Option C:** IgM is a large molecule that does not cross the placenta and is primarily involved in the ABO blood group antigen reaction.

**Clinical Pearl / High-Yield Fact**
The ability of IgG1 to cross the placenta and activate complement highlights the importance of maternal IgG1 levels in protecting against infections during the perinatal period. This underscores the significance of adequate maternal vaccination and the potential risks associated with immunoglobulin deficiency in pregnancy.

**Correct Answer: A. IgG1. This immunoglobulin crosses placenta readily and can activate complements.**