Answer: Going on a walk is not a run-on sentence. It's a sentence fragment.
Explanation: I don't believe I can see the answer choices, but I can confirm that 'going on a walk' is a sentence fragment. A run-on sentence example is: It was very nice outside the boy flew a kite. You would need a semi-colon or a period. Like this: It was very nice outside; the boy flew a kite
Hope this helps. :)
- Your Friendly Neighborhood 8th Grader ._.