You are developing a client application that consumes a Windows Communication Foundation (WCF) service.
You use the svcutil.exe utility to create a proxy for the service. You use the svcutil.exe switches that generate asynchronous calls.
GetFlight is a service operation that takes no parameters and returns a string.
The GetFlightCallback method must be called when the service operation returns.
You create an instance of the client proxy with the following code:

var client = new TranvelServiceClient();

You need to ensure that a callback is received when the GetFlight operation is called asynchronously. Which code segment should you use?

A.
client.BeginGetFlight(GetFlightCallback, null);
client.GetFlight();

B.
client.GetFlight();
client.BeginGetFlight(GetFlightCallback, null);

C.
client.GetFlightCompleted += new EventHandler<GetFlightCompletedEventArgs>(GetFlightCallback);
client.GetFlightAsync();

D.
IAsyncResult asyncResult = client.BeginGetFlight(GetFlightCallback, client);
client.EndGetFlight(asyncResult);

Explanation:
// AddAsync
double value1 = 100.00D;
double value2 = 15.99D;
client.AddCompleted += new EventHandler<AddCompletedEventArgs>(AddCallback);
client.AddAsync(value1, value2);
Console.WriteLine(“Add({0},{1})”, value1, value2);

How to: Call WCF Service Operations Asynchronously
(http://msdn.microsoft.com/en-us/library/ms730059.aspx)