## 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. The API is calculated using the number of confirmed malaria cases and the population at risk.

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

## Why Each Wrong Option is Incorrect
- **Option A:** Without a specific value provided, we cannot directly assess its correctness, but based on the calculation, it does not match our result.
- **Option B:** Similarly, without a specific value, we cannot assess its correctness directly, but our calculation yields an API of 1.
- **Option C:** This option suggests an API of 2, which would imply 200 confirmed cases for the same population, not matching our given data.
- **Option D:** This option is not provided with a specific value in the query, but based on the correct calculation, the accurate API is 1.

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

## Correct Answer: B. 1