Explanation:

- Option 1: Scale

- "Scale" refers to the size or level of something, but it isn't commonly used with growth contexts in population discussions.

- Option 2: Rate

- The term "rate" is appropriate here as it refers to the speed or frequency at which an event occurs, such as population growth. This fits well in the context of a population settling down to zero growth.

- Correct

- Option 3: Frequency

- "Frequency" refers to how often something happens over a particular period. It doesn’t fit well with the idea of growth in a population context.

- Option 4: Degree

- "Degree" refers to the extent or amount of a phenomenon. While related, it’s not the best fit compared to "rate" when discussing population growth.