**Core Concept**
The question is testing the knowledge of immunoglobulins present in breast milk and colostrum, which provide passive immunity to the newborn. Immunoglobulins are a type of antibody that play a crucial role in the immune system.

**Why the Correct Answer is Right**
The most common immunoglobulin secreted by the mother in milk and colostrum is IgA. IgA antibodies are produced by B cells in the mother's gut and are secreted into the gut lumen, where they bind to pathogens and prevent their attachment to the intestinal lining. These IgA antibodies are then transported to the mammary glands and secreted into breast milk, providing protection to the newborn against infections.

**Why Each Wrong Option is Incorrect**
**Option A:** IgM is the first antibody produced in response to an infection and is not typically found in breast milk.
**Option B:** IgE is involved in allergic reactions and is not the primary immunoglobulin secreted in breast milk.
**Option C:** IgG is the most common antibody in the blood, but it is not the primary immunoglobulin secreted in breast milk.

**Clinical Pearl / High-Yield Fact**
It's worth noting that the IgA antibodies in breast milk are specific to the pathogens that the mother has been exposed to, which makes breast milk a valuable source of passive immunity for the newborn.

**Correct Answer: D. IgA.**