If immigration and emigration numbers remain equal, which of these could cause a slowed
growth rate?

increased birthrate
constant death rate
decreased birthrate
constant birthrate

If immigration and emigration numbers remain equal, decreased birthrate could cause a slowed growth rate.

Explanation:

In demographics, the term birth rate of a population is the number of births per 1,000 people each year. The results of this calculation are combined with the death rate to produce a rate of natural population growth rate (naturally meaning not to involve a population movement rate -migration-).

Birthrates tend to be higher in countries with less developed economies and lower in countries with high economic growth.