The birthrate is a measurement that calculates the number of births that occur within a specific time frame, usually a year, per a certain population size. In this question, the correct option is option 2, which states a birthrate per 1000 population. This means that for every 1000 individuals within a given population, the number of births that occur in a year is measured. It is important to note that the birthrate is usually expressed as a ratio or a rate per a specific population size, as it allows for easier comparison across different population sizes and demographics. For example, comparing the birthrate between two countries with different population sizes would be more accurate if it is measured per a consistent unit, such as per 1000 people.