In the sentence you provided, "We are excited for summer vacation for we have plans to visit several national parks," the relevant grammatical rule is related to A: a long introductory element.
Although the introduction in this particular sentence isn't lengthy, it does set up the main clause and could be considered an introductory element. However, the sentence structure might not strictly require a rule to be applied since there's no specific introductory phrase that needs a comma.
If you were to modify the sentence to create a longer introductory element, it could look something like this: "Excited for summer vacation, we have plans to visit several national parks." In this case, you would definitely use a comma after the introductory phrase.
Let me know if you need more information or further examples!