Final answer:
The correct compound sentence is 'Eva was a kid, but she wasn’t loud or boring,' because it uses the coordinating conjunction 'but' to link two main clauses and convey a contrast.
Step-by-step explanation:
The sentence that best combines sentences 12 and 13 into a compound sentence for sentence variety is: Eva was a kid, but she wasn’t loud or boring. This sentence uses a coordinating conjunction (‘but’) to join two main clauses, effectively adding complexity and depth to the writing.
When combining sentences for a more effective sentence structure, it's important to ensure that the connection between the ideas is maintained and the overall meaning is preserved. The use of ‘but’ clearly conveys the contrast between Eva being a child and not displaying typical childlike traits, which is the intended meaning.