Final answer:
The key feature of messaging not supported in WCF is asynchronous communication with an offline receiver. WCF requires both parties to be online for message exchange, though integration with systems like MSMQ can provide offline messaging capabilities.
Step-by-step explanation:
The key feature of a messaging system that is not supported in Windows Communication Foundation (WCF) is c. Asynchronous communication with an offline receiver. WCF supports both synchronous and asynchronous communication models when both the sender and receiver are online. However, WCF does not inherently support a durable messaging system where messages can be sent to an offline receiver to be retrieved once the receiver comes back online. This feature is typically provided by a message queuing system, such as MSMQ (Microsoft Message Queuing), which can be integrated with WCF, but is not a native feature of WCF itself.