The answer is: c. dreary.
The tone of a written work is the attitude of the author towards the subject. A dreary tone expresses a somber, gloomy perspective.
In "Le Morte D'Arthur," the writer narrates the story in a cheerless, dismal manner, creating a general feeling of mortality before Arthur dies.
The rest of the options are incorrect because the composition does not include neither irony or language to mock the characters for their actions, nor an educational or instructional point of view.