The most accurate response that describes how the demographic transition model can be used to study changing populations, based on the provided graph of natural increase rates (NIR) for different regions, is:
The regional total fertility rates are observable; this data can then be compared to the population growth rates.
This response makes sense because total fertility rates are a crucial factor influencing population growth, which is reflected in the natural increase rates shown in the graph. In developing regions, higher fertility rates typically contribute to higher rates of natural increase, while in developed regions like North America, lower fertility rates lead to lower population growth rates.