Introduction:
When the growth rate of a population is decreasing and the population is reaching towards saturation, it is important to use a method that can effectively capture this trend and provide accurate predictions. Among the given options, the most suitable method in this scenario is the "Decreased rate of growth method."

Explanation:
The "Decreased rate of growth method" is a statistical technique that takes into account the decreasing growth rate of a population. It considers the trend of the growth rate and uses it to make predictions about future population sizes. This method is particularly useful when the population is approaching saturation because it acknowledges that the growth rate cannot continue to increase indefinitely.

Advantages of the Decreased rate of growth method:
1. Captures the trend: This method takes into account the decreasing growth rate, which is essential when the population is approaching saturation. It accurately reflects the slowing down of population growth.
2. Provides realistic predictions: By considering the decreasing growth rate, this method provides more realistic predictions about the future population sizes. It acknowledges that the growth rate cannot continue to increase indefinitely and adjusts the predictions accordingly.
3. Helps in planning: Understanding the future population sizes is crucial for planning purposes. The decreased rate of growth method helps in making informed decisions about resource allocation, infrastructure development, and other planning activities.

Other methods:
1. Arithmetical increase method: This method assumes a constant growth rate over time. It is not suitable when the growth rate is decreasing as it does not capture the trend accurately.
2. Graphical comparison method: This method involves comparing population data from different time periods using graphs. While it can provide insights into the population trend, it may not be as accurate as the decreased rate of growth method in capturing the decreasing growth rate.
3. Zoning method: The zoning method involves dividing a region into different zones based on population density. While it can be useful for spatial planning, it may not be as effective in predicting future population sizes when the growth rate is decreasing.

Conclusion:
In summary, when the growth rate is decreasing and the population is reaching towards saturation, the most suitable method is the "Decreased rate of growth method." This method acknowledges the decreasing growth rate and provides more realistic predictions about future population sizes. It is valuable for planning purposes and helps in making informed decisions about resource allocation and infrastructure development.