You are developing a WCF service.
A new service instance must be created for each client session.
You need to choose an instancing mode.
Which instance mode should you use?

A.
PerCall

B.
Single

C.
Multiple

D.
PerSession

E.
PerRequest

Explanation:
PerSession: A new InstanceContext (and therefore service object) is created for each new client session andmaintained for the lifetime of that session (this requires a binding that supports sessions).
Incorrect: Answers
A: PerCall: A new InstanceContext (and therefore service object) is created for each client request.
B: Single: A single InstanceContext (and therefore service object) handles all client requests for the lifetime of
the application.
https://msdn.microsoft.com/en-us/library/ms731193(v=vs.110)