Answer: The bolded words ("not only...but also") form a correlative conjunction.
Step-by-step explanation: Correlative conjunctions are conjunctions made up of two parts, that is to say they occur in pairs, and they are used to connect ideas that have equal importance. In the sentence provided, "not only...but also" is a correlative conjunction since it is balancing two ideas: that Evan's joke was silly and that Evan's joke was too long. Other examples of correlative conjunctions are neither/nor and either/or.