**Core Concept**
The question is testing the concept of disease prevention and the cost-effectiveness of different interventions. The correct answer is related to the idea of primary prevention, which aims to prevent the occurrence of disease before it occurs. This can be achieved through various means, including vaccination, health education, and environmental modifications.

**Why the Correct Answer is Right**
The correct answer is likely to be **C. Vaccination**. Vaccination is a highly effective and cost-effective mode of intervention for preventing diseases. It works by stimulating the body's immune system to produce antibodies that can recognize and fight specific pathogens. This can prevent the spread of infectious diseases and reduce the burden on healthcare systems. Vaccination has been shown to be more cost-effective than other forms of disease prevention, such as screening and treatment.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is likely to be incorrect because it is too vague and does not specify a particular intervention. While health education is an important aspect of disease prevention, it is not the most cost-effective mode of intervention.
**Option B:** This option is likely to be incorrect because it is too broad and does not specify a particular type of intervention. While screening and treatment are important aspects of disease management, they are not the most cost-effective modes of intervention for preventing disease.

**Clinical Pearl / High-Yield Fact**
One key point to remember is that vaccination is a highly effective and cost-effective mode of intervention for preventing diseases. It is estimated that vaccination prevents 2-3 million deaths worldwide each year, making it one of the most effective public health interventions available.

**Correct Answer:** C. Vaccination