About Drupal interact with Android Requirements:~


Android app post json data to a certain url.
Drupal can  receive the json data and then verify the json data and return result data in json format.


My question is on receive Json data and return Json data steps. What module or api can I use?

It would be greater help if you can share a sample.
Services module is what you are looking for. It supports json data. More about the module:


  A standardized solution of integrating external applications with Drupal. Service callbacks may be used with multiple interfaces like REST, XMLRPC, JSON, JSON-RPC, SOAP, AMF, etc. This allows a Drupal site to provide web services via multiple interfaces while using the same callback code. 


It also has an extensive documentation.