Final answer:
A time series graph, specifically a line graph, is the best descriptive chart for analyzing the relationship between weekly sales of air-conditioning units and the time of year, as it clearly shows trends and patterns over time.
Step-by-step explanation:
A researcher interested in determining whether there is a relationship between the weekly number of room air-conditioning units sold and the time of year would benefit most from using a time series graph. This type of graph is ideal for displaying data points over time and is particularly useful when the variables are not random but are ordered chronologically.
A line graph as a format for a time series graph would be most effective in this case, showcasing how the number of air-conditioning units sold varies as time progresses throughout the year. This graphical representation can illustrate patterns, trends, or seasonal effects on air-conditioning unit sales over time.