The protective effects off breast milk are known to be associated with –
Although antibodies and Lysozyme, both are important for the protective effects of breast milk, antibodies play a major role. Out of IgM and IgA antibodies, breast milk is especially rich in secretory IgA which is the single best answer here.
Protective role of breastfeeding against infection 

Breast milk provides protection against infection because of the following facts -

1. The breast milk is clean and uncontaminated
2. Breast milk contains several antiinfective factors -

Antibodies → secretory IgA, IgM
Lysozyme
Antistaphylococcal factor
Specific inhibitory substances against viral infections.
Lactoferrin → Inhibits growth of E. coli.
Bile stimulated lipase → kills Entamoeba histolytica and Giardia lamblia.
Bifidus factor → Inhibits growth of E. coli
Para-amino-benzoic acid (PABA) → Provides protection against malaria
Phagocytic macrophages and lymphoid cells

3. Lower pH of the stool of breastfed infants contributes to the favourable intestinal flora more bifidobacteria and lactobacilli, fewer E. coli.
IgM antibodies
Lysozyme
Mast cells
IgA antibodies