**Core Concept**
Herd immunity refers to the protection of a community from an infectious disease when a sufficient percentage of the population is immunized, thereby preventing the spread of the disease. This concept is crucial in disease elimination, as it reduces the number of susceptible individuals who can contract and spread the disease.

**Why the Correct Answer is Right**
Herd immunity is achieved when a sufficient percentage of the population is immunized, typically around 80-90% for most diseases. This threshold is often referred to as the "herd immunity threshold." When a large enough percentage of the population is immunized, it creates a barrier that prevents the spread of the disease, thereby protecting those who are not immunized, such as young children or individuals with weakened immune systems. This is particularly important in disease elimination, as it reduces the number of cases and prevents the disease from spreading to new areas.

**Why Each Wrong Option is Incorrect**
**Option B:** Isolation involves separating an infected individual from the general population to prevent the spread of the disease. While isolation is an important control measure, it is not the primary factor that contributes to disease elimination.
**Option C:** Quarantine involves restricting the movement of individuals who have been exposed to a disease, but have not yet developed symptoms. Like isolation, quarantine is an important control measure, but it is not the primary factor that contributes to disease elimination.
**Option D:** None is incorrect because herd immunity is a key factor in disease elimination.

**Clinical Pearl / High-Yield Fact**
A useful mnemonic to remember the herd immunity threshold is "80% or more, that's the score." This reminds us that a sufficient percentage of the population needs to be immunized to achieve herd immunity.

**✓ Correct Answer: A. Herd immunity**