Which is the default Message Exchange Pattern (MEP) ?

Below are 6 type of MEP and
1)Datagram
2)Request-response
3)Duplex
4)Datagram with sessions
5)Request-response with sessions
6)Duplex with sessions

Request-response is the default MEP because it supports both an operation in which one or more parameters are passed to the operation and a return and one or more out values that the operation passes back to the caller.