It's called a Notifier.
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Then how can i make it is there any example aia available?
Read the documentation and try the ShowChooseDialog notifier
http://ai2.appinventor.mit.edu/reference/components/userinterface.html#Notifier
A very good way to learn App Inventor is to read the free Inventor's Manual here in the AI2 free online eBook App Inventor 2 Book: Create Your Own Android Apps ... the links are at the bottom of the Web page. The book 'teaches' users how to program with AI2 blocks.
There is a free programming course here Course In A Box and the aia files for the projects in the book are here: App Inventor 2 Book: Create Your Own Android Apps
How to do a lot of basic things with App Inventor are described here: How do you...? .
Also do the tutorials Our Tutorials! to learn the basics of App Inventor, then try something and follow the Top 5 Tips: How to learn App Inventor
Taifun
Trying to push the limits! Snippets, Tutorials and Extensions from Pura Vida Apps by 
Taifun.
that screenshot i had uploaded 2 days ago in that screen has a pop up screen in which the phone is asking something before execution, that pop up screen has buttons
similarly can i make that kind of pop screen that has buttons , is it possible to build it in mit app inventor with the notifier.
Yes
Taifun
Which of the Notifier blocks have you tried so far?