Final answer:
The sentence in question is a complete sentence, not a run-on, because it uses the coordinating conjunction 'and' to join two actions with the same subject correctly.
Step-by-step explanation:
The sentence 'They danced and said goodnight.' qualifies as a complete sentence and not a run-on. A run-on sentence typically consists of two or more independent clauses (complete sentences) that are not properly connected or punctuated. However, in this case, the sentence employs a coordinating conjunction 'and' to properly join two actions related to the same subject. This creates a compound predicate, which is grammatically correct and does not constitute a run-on. Thus, the correct answer is: 1) Sentence.