Herd immunity is the resistance against a contagious disease provided by the immune individuals which makes the major part of the population to few susceptible individuals in the community.
So as most of the population is immune to the disease therefore that disease is not able to overpass these immune individuals and the susceptible person is protected indirectly from that contagious disease. By vaccinating most of the people against a disease we can gain herd immunity. So the right answer is herd immunity.
