Final answer:
A Line graph is the best option for displaying the population of a country as it changes each month, as it clearly shows trends over time. Bar graphs are better for single-time comparisons and not suited for showing continuous data trends like monthly population changes.
Step-by-step explanation:
The best type of graph for showing how the population of a country changes each month would be a Line graph. Line graphs are particularly adept at displaying data trends over time and can easily illustrate the rise and fall in population numbers with clarity. As months progress, each data point on the line graph would reflect the population at that specific time, providing a continuous and comprehensible visual of the population change.
In contrast, a Bar graph is more suitable for comparing quantities at a specific point in time rather than showing continuous change. Bar graphs represent data with distinct bars, making them ideal for comparing the populations of different countries or different groups within a population at a single instance, but less effective for showing data trends over a period such as monthly changes.