Which message pattern should you choose?

You are developing a WCF service that compares several data sources. The service takes a long time to
complete.
The service must meet the following requirements:
 The client must be able to continue processing while the service is
running.
 The service must initiate communication with the client application
when processing is complete.
You need to choose a message pattern to meet the requirements.
Which message pattern should you choose?

A.
One Way

B.
Streaming

C.
Duplex

D.
Request/Reply