The shortcut key to insert a new slide is:

Keyboard shortcuts are essential for efficient navigation and editing in Microsoft PowerPoint. The question asks for the shortcut key combination used to insert a new slide into an existing presentation.

• Ctrl + M is the correct answer. This shortcut is specifically designed to insert a new blank slide into your current presentation, typically using the default title and content layout.
• Ctrl + N is incorrect. This shortcut is used to create a *new, blank presentation* entirely, not just a new slide within the active presentation.
• Ctrl + S is incorrect. This is a universal shortcut across most applications, including PowerPoint, used to *save* the current presentation.
• Ctrl + I is incorrect. This shortcut is commonly used to apply *italic* formatting to selected text within a slide.

Using Ctrl + M significantly speeds up the process of building out a presentation slide by slide.