Concept:Newborn calves gain natural passive immunity from antibodies present in the mother's first milk, called colostrum.
Explanation:Colostrum is the first milk produced by the mother immediately after calving.
It is rich in maternal antibodies, especially immunoglobulins.
When a calf drinks colostrum soon after birth, these antibodies are absorbed into its bloodstream.
This provides immediate natural protection before the calf's own immune system matures.
A vaccine would create artificial immunity, not natural immunity.
A balanced diet and salt licks support general growth and health, but they do not supply ready-made antibodies.
Therefore, colostrum is the natural source of early immunity in calves.
Answer:B. Colostrum