The correct answer is A. .cls.
A class module is a type of module that contains a class definition. A class is a blueprint for creating objects. Objects are instances of classes. Class modules are used to create reusable code.
The extension of a class module is .cls. This extension is used by the Visual Basic development environment to identify class modules.
The other options are incorrect.
Option B, .frm, is the extension of a form. A form is a user interface object that is used to interact with the user.
Option C, .pvb, is the extension of a VBA project file. A VBA project file is a file that contains one or more modules.
Option D, .mdb, is the extension of a Microsoft Access database file. A Microsoft Access database file is a file that contains data and objects.