Step 1: Understand colostrum. Colostrum is the thick yellowish milk a mother makes in the first few days after the baby is born.
Step 2: Why it matters. A newborn's own immune system is still weak. Colostrum is packed with antibodies that protect the baby for the first weeks. This is a kind of passive immunity given by the mother.
Step 3: Which antibody is in it. The main antibody in colostrum is IgA. It coats the lining of the baby's gut and guards against germs that enter through food.
Step 4: Why not the others. IgG mainly crosses the placenta before birth. IgM is the first antibody made during an infection. IgE is linked to allergies and parasite defence. None of these is the chief antibody in colostrum.
Step 5: Conclusion. Therefore the correct answer is "Ig A". \[ \boxed{\text{Ig A}} \]