Hi Sachin - welcome to the community 
It’s a very long time since I have used Nant, but I will try and help!
The only way to analyse .NET applications is using the msbuild option. I presume that your Nant build contains msbuild commands? If so you should select the ‘integrate the with msbuild’ option. As long as there is a build running between your ‘prepare task’ and ‘run code analysis’ you should see a result.
Tom