The shortcut key used to save the document in MS Word 365 is:

Correct option is A

Ctrl + S is the standard keyboard shortcut for saving a file in Microsoft Word 365, as well as many other applications across different operating systems. Using this shortcut quickly saves the current document. If it's a new document, it will prompt you to choose a file name and location. If it's an existing document, it will save the changes to the current file.  
Important Key Point:
1. Ctrl + S is a crucial shortcut for preventing data loss. It's good practice to save your work frequently while editing a document.
2. This shortcut is consistent across many programs, making it easy to remember and use efficiently.
Knowledge Booster:
· Ctrl + A is the shortcut for "Select All," which selects all the content within the current document.
· Ctrl + X is the shortcut for "Cut," which removes the selected content and places it on the clipboard for pasting elsewhere.
· Ctrl + V is the shortcut for "Paste," which inserts the contents of the clipboard into the current document at the cursor's location.