Which immunoglobulin is able to cross the placenta?

The immunoglobulin that is capable of crossing the placenta is IgG. This property is crucial because it allows for the transfer of maternal antibodies to the fetus, providing the newborn with passive immunity against infections during the first few months of life. IgG is the only antibody class that can traverse the placental barrier, contributing to the development of the infant's immune system.

IgA primarily functions in mucosal immunity and is found in high concentrations in secretions like breast milk, but it does not cross the placenta. IgD is less understood but is mainly involved in the activation and regulation of B cells and does not cross the placenta. IgM, although an effective antibody in the primary immune response, is the largest antibody and cannot efficiently cross the placental barrier either. Thus, IgG's ability to cross provides vital protective antibodies to the fetus, making it the correct answer to the question.