Answer : 3. "ctrl+down arrow "
Explanation :

Actually, in MS PowerPoint (and most other word processing applications), the key combination to move down one paragraph is "Enter". Pressing the "Enter" key moves the cursor to the next paragraph. If you want to move the cursor down one line within the same paragraph, you can use the "Down Arrow" key. There isn't a specific default key combination to move down one paragraph in PowerPoint; it's more about using the "Enter" key to start a new paragraph.