Final answer:
The best method to determine cereal sales in a given region is to collect supermarket scanner data.
Step-by-step explanation:
The best method for a researcher to determine how many boxes of a particular brand of cereal were sold in a given region in a typical week is to collect supermarket scanner data.
This method involves collecting data from the supermarket scanners which automatically record sales at the point of purchase. This data provides accurate and real-time information about the number of boxes of cereal sold in the region. It eliminates the need for manual surveys or observations, resulting in a more reliable and efficient method of data collection.