You have finished . . . . . . . . last.

In the sentence "You have finished...last," the correct preposition to use is at.

So, the complete sentence is: "You have finished at last."

The preposition "at" is used to specify a particular point in time or a specific moment when something happened or was completed. In this sentence, "at last" means that you completed something after a period of time or effort, emphasizing the sense of relief or accomplishment.

Therefore, the correct answer is Option D: at.