Final answer:
eBay is not a peer-to-peer model as it acts as an intermediary for buyers and sellers, contrasting with Instant Messaging, which enables direct communication between users.
Step-by-step explanation:
The question asks which of the following is not a peer-to-peer model: A. eBay or B. Instant Messaging. In the context of network architectures, a peer-to-peer (P2P) model involves a decentralized network where each participant (peer) has equal capabilities and either may initiate a communication session. Instant Messaging is typically considered a peer-to-peer application because messages can be sent directly between users without the need for a central server once a connection is established. On the other hand, eBay does not operate on a peer-to-peer model; instead, it is an online auction and shopping website where the company provides a platform for buyers and sellers to transact, but itself acts as an intermediary rather than a direct peer.
Therefore, eBay is not a peer-to-peer model.