Final answer:
A fetus infected with rubella virus produces IgM antibodies that can be detected in the umbilical cord. IgM is the first class of antibodies to be produced in response to an infection.
Step-by-step explanation:
The correct answer is option B. IgM. When a fetus is infected with the rubella virus, it produces IgM antibodies that can be detected in the umbilical cord. IgM is the first class of antibodies to be produced in response to an infection, and its presence suggests an acute or recent infection.
IgA antibodies are primarily found in secretions such as saliva and mucous, and they play a role in protecting the body's mucosal surfaces.
IgG antibodies are the most abundant class of antibodies and are involved in long-term immunity, as they can cross the placenta and provide passive immunity to the fetus.
IgD antibodies are involved in the activation of B-cells and are less relevant to this specific scenario.