The correct answer is the following: option D. The use of the word "joke"is the one that most clearly makes the tone of the sentence inappropriate for a class presentation. When formally writing an essay or a story for a class it is important to give a good impression through your writing. To do so it is recommended to use a more formal speech than the one you would commonly used when speaking to friends or family members. That is why the use of slang or colloquial terms such as "joke" is seen as an inappropriate tone for a class presentation.