The right answer is A. active immunity.
Active immunization is another way of referring to vaccination. It is the perfect illustration of the old saying that "prevention is better than cure". This method of prevention is said "active" because it consists in stimulating the immunity of an individual even before contracting the disease.
The father of active immunization is a Edward Jenner. In 1788, he was the first to observe in cowpox boys with cowpox (bovine smallpox) that they were actually immune to human smallpox.
