Which of the following describes file extensions?

File extensions serve as an identifier that specifies the format of a file. They are typically a series of characters added to the end of a filename, following a period, and they indicate the type of content or the program that can open the file. For instance, a file with a ".txt" extension is recognized as a text file, while a ".jpg" extension indicates an image file. This identification is crucial for operating systems and applications to determine how to handle the file appropriately, ensuring correct opening and processing.

The other options do not accurately describe file extensions. A file size is denoted by different metrics, and creation dates are not indicated by prefixes in filenames. Furthermore, file compression techniques involve different processes and formats, which are not described simply by file extensions. Thus, the correct answer emphasizes the role of file extensions as vital identifiers of file formats used across various computing environments.