(d) IgA

Explanation:

Colostrum (also called first milk) is the thin yellowish fluid secreted by the mammary glands at the time of parturition and which precedes the production of true milk. It provides a nursing infant with essential nutrients and infection-fighting antibodies (called immunoglobulin). I_gA is the major immunoglobulin in colostrum. It provides protection from inhaled and ingested pathogens.