The conjunction that should fill in the blank in this sentence is the word OR. So, you can retake the test tomorrow or you can do some extra-credit work, These types of conjunctions, called correlative conjunctions, always go in pairs, such as either-or, neither-nor, not only-but also, etc. So, the conjunction is used to show that a person has to choose between these two things. Nor cannot be used because it is a negative conjunction. But and and do not fit here.