## Core Concept
The Annual Parasite Incidence (API) is a measure used to express the number of confirmed malaria cases per 1000 population per year. It is an important epidemiological tool for assessing the malaria situation in a given area.

## Why the Correct Answer is Right
To calculate the API, we use the formula: API = (Number of confirmed malaria cases / Total population) * 1000. Given that there were 100 confirmed malaria cases (slides positive for malarial parasite) and a population of 100,000, we can substitute these values into the formula. Therefore, API = (100 / 100,000) * 1000 = 1.

## Why Each Wrong Option is Incorrect
- **Option A:** This option is incorrect because it does not match the calculation for API based on the given data.
- **Option B:** Similarly, this option does not correspond to the correct calculation of API.
- **Option D:** This option is also incorrect as it does not align with the API value calculated from the provided numbers.

## Clinical Pearl / High-Yield Fact
A key point to remember is that API is a crucial indicator for malaria control programs, reflecting the malaria transmission intensity in a specific area. An API of 1 means there is 1 case of malaria per 1000 population per year, which can guide public health interventions.

## Correct Answer: B. 1.