Answer: B. There is a run-on. To fix it, replace the comma with a semi-colon.
Explanation: This sample sentence is comprised of two independent clauses. They're fine on their own (both are complete sentences), but here they are connected by a comma. It's a run-on. You can either replace the comma with a semi-colon or throw in a conjunction after the comma (such as "so"). 'She is tired, so I'm putting her down for a nap' makes sense, but this is not an option. So here, the correct option would be B!
"She is tired; I am putting her down for a nap."