To answer this question, you need to understand the file extensions used in web development.
Option A) .Asmx - This option is incorrect because the .asmx extension is used for ASP.NET web services, not web user control files.
Option B) .Ascx - This option is correct. The .ascx extension is used for web user control files in ASP.NET. These files contain reusable UI components that can be added to web pages.
Option C) .Aspx - This option is incorrect because the .aspx extension is used for web forms in ASP.NET, not web user control files.
The correct answer is B) .Ascx. This option is correct because it represents the file extension of a web user control file in ASP.NET.