Final answer:
The correct appositive phrase is "The California coastline, a national treasure, is the longest of all states," which includes an appositive that provides additional information about the subject.
Step-by-step explanation:
The correct appositive phrase that combines the two sentences about the California coastline is: The California coastline, a national treasure, is the longest of all states. An appositive is a noun or noun phrase that renames or clarifies another noun next to it. In this case, "a national treasure" is the appositive that provides additional information about "The California coastline." It's important to note that while the Baja California Peninsula is a geographical feature mentioned in LibreTexts™, it is separate from the California coastline referenced in the question.