Which of the following is an example of an auction site that operates under a consumer-to-consumer business model?

The correct answer is eBay because it is a prime example of a consumer-to-consumer (C2C) business model. In this model, individual users can buy and sell goods to one another through the auction format that eBay facilitates. Users list items for sale, set starting prices, and others place bids, creating an interactive marketplace focused on direct consumer transactions.

The success of eBay's C2C model lies in its platform that allows individuals to reach a wide audience of potential buyers without needing to own inventory or have a physical storefront. This model has empowered everyday people to engage in commerce, turning them into sellers and creating a dynamic marketplace driven by supply and demand.

Other choices like Amazon and Alibaba primarily operate on a business-to-consumer (B2C) model, where a company sells to consumers. Facebook Marketplace operates C2C similar to eBay, but eBay is more widely recognized specifically for its auction-style listings, making it the quintessential example of a C2C auction site.