Hi Raj,

In my opinion, the Activity Diagram would include the basic scenario and all major alternative scenarios.

A Use Case description doesn't have to include an Activity Diagram, it's optional. That being said, if you have a rather complex Use Case with lots of branching alternative scenarios, then that's a particular instance in which you'd find it useful to include one.

There's an IBM article on mapping Use Cases to Test cases which mentions the inclusion of Activity Diagams and branching alternative scenarios:

http://www.ibm.com/developerworks/rational/library/04/r-3217/

Hope that helps.

Graham