Answer:
The second sentence is a run-on sentence
Explanation: A run-on sentence is when two or more independent clauses (phrases that can stand on their own as a sentence) are not connected properly. You can connect them with a comma, or a conjunction(and, so, etc)
The second sentence has two independent clauses, "you must be going somewhere nice" and "your outfit is so elegant". However, these aren't connected properly so the answer is the second choice.
