What must be scanned before processing a purchase transaction at Costco?

The membership card must be scanned before processing a purchase transaction at Costco because it serves as proof of membership eligibility. Costco operates on a membership-only model, which means that customers are required to pay an annual fee to gain access to the store and its services. Scanning the membership card verifies that the individual is an active member, thereby allowing them to make purchases.

This practice is essential for maintaining the integrity of Costco’s business model, as it ensures that only those who have paid for membership can shop there, which contributes to the store's revenue and membership-based pricing structure. The other options, while related to important aspects of shopping and employment at Costco, do not fulfill the primary requirement for transaction processing.