What does the Windows operating system examine to determine which application should be used to open a file?

The Windows operating system primarily examines the file extension to determine which application should be used to open a file. The file extension is the suffix at the end of a filename, typically consisting of three or four characters that indicate the file type. For example, a file with a ".txt" extension is recognized as a text file, which Windows will associate with a text editor like Notepad. Similarly, a ".jpg" extension indicates an image file, leading Windows to use an image viewer.

While the file size, file name, and file permissions are factors that can play roles in file management and access control, they do not directly influence the selection of the application for opening the file. The file size may indicate how many resources an application might need, while the file name is primarily a label, and permissions dictate access rights, none of which serve the primary function of associating a file with an application for opening. Hence, the file extension is the key element that enables the operating system to identify the appropriate application for opening a given file.