Final answer:
The default file extension for saving a presentation in PowerPoint 365 is .pptx. This format provides reduced file size and improved data recovery, compared to other formats like .ppt, .pptm, and .pps.
Step-by-step explanation:
When you save a presentation in PowerPoint 365, the default file extension applied is .pptx. This format is known as PowerPoint Open XML, which is a newer format introduced with Microsoft Office 2007 that provides benefits such as reduced file size and improved data recovery.
Other file extensions like .ppt, .pptm, and .pps are also associated with PowerPoint but serve different purposes; .ppt is the older format for PowerPoint presentations, .pptm indicates a macro-enabled presentation, and .pps is used for PowerPoint slide shows.