The sentence "We are excited for summer vacation, for we have plans to visit several national parks" uses the coordinating conjunction "for."
In this sentence, "for" functions to explain the reason behind the excitement. It's used to introduce a clause that provides justification or explanation for the statement made in the first part of the sentence. The rule being applied here is that coordinating conjunctions (such as "for," "and," "nor," "but," "or," "yet," and "so") can connect independent clauses and indicate the relationship between them. In this case, "for" indicates a cause-and-effect relationship.