What does a file extension signify in a filename?

A file extension is a suffix at the end of a filename, typically comprising a dot followed by a few letters. It serves as a key indicator of the type of data contained within that file. For instance, a file with a ".docx" extension signifies that it is a Microsoft Word document, while a ".jpg" extension indicates that the file is a JPEG image. This helps both users and operating systems to recognize the format and determine which software is suitable for opening or manipulating that specific type of file.

While the other options relate to certain attributes of files, they do not correctly capture the role of a file extension. The file extension does not determine the file's size, nor does it provide information about the date of creation or the specific software required to open it. Instead, its primary function is to denote the file's data type, facilitating proper handling and usage in various software applications.