Herd immunity refers to:

Correct option is C

Herd immunity occurs when sufficient individuals are immune (by vaccination or infection), making disease transmission unlikely, protecting even those not immune.
Explanation of options: (a) ✘ That is passive maternal immunity, not herd immunity. (b) ✘ Can be via vaccination also, not only natural infection. (c) ✔ Correct; reduced transmission occurs when critical population fraction is immune. (d) ✘ Boosters may contribute, but not the definition.