Step 1: Understand the question.
We need the shortcut key that exits a running PowerPoint presentation.
Step 2: Picture Slide Show mode.
When a show runs, slides fill the whole screen. We need a quick key to stop it and return to editing.
Step 3: Role of the Esc key.
The Escape key, marked Esc, instantly stops the slide show and brings you back to the normal editing view.
Step 4: Check Backspace and Del.
Backspace moves to the previous slide during a show, and Del deletes objects in editing. Neither exits the show.
Step 5: Check Ctrl plus Enter.
Ctrl plus Enter does other jobs like moving to placeholders, not ending the presentation.
Step 6: Conclude.
So the exit key for a presentation is Esc, which is option 1.
\[ \boxed{\text{Esc}} \]