To answer this question, the user needs to know the common file extensions used in ASP.NET development.

The correct answer is:

B. .ASPX

Option A (.ASP) is incorrect because it is the file extension used for Classic ASP files, not ASP.NET files.

Option C (.Web) is incorrect because this is not a valid file extension used in ASP.NET development.

Option D (None of the above) is incorrect because .ASPX is, in fact, the correct file extension used for ASP.NET files.

Therefore, the answer is: B. .ASPX