Step 1: Concept
Passive immunity provided by breast milk (colostrum).
Step 2: Analysis
Assertion (A) is true: Breastfeeding is vital for infant health. Reason (R) is true: Colostrum, the yellowish fluid produced in the initial days, is rich in IgA antibodies, providing passive immunity to the newborn. This immune protection is a primary reason why breastfeeding is healthy for the baby.
Step 3: Conclusion
Both are true and (R) explains (A).
Final Answer: (A)