Final answer:
Retailers can analyze check-in data to understand customer preferences and demographics, improving their marketing strategies.
Step-by-step explanation:
The retailer can best use check-in data to gather information about the customers by leveraging option B: Use check-in data to track customer preferences and demographics. This type of data can provide invaluable insights into customer behaviour, such as how often they visit, which days they prefer to shop, and potentially what products they may be interested in based on the sections of the store they frequent. Moreover, the retailer could correlate check-in times with purchase data to gain a better understanding of the consumer's shopping habits and preferences and tailor marketing strategies accordingly, such as targeted promotions or personalized recommendations.