The correct answer is Chaucer because Matthew Arnold, despite recognizing Chaucer's literary importance, did not regard him as possessing the same level of "high poetic seriousness" that he believed characterized the great classical poets, such as Homer, Dante, Shakespeare, or Milton.
In his essay "The Study of Poetry", Arnold discusses the concept of poetry as a "criticism of life" and emphasizes the importance of seriousness and moral depth. Although Arnold appreciated Chaucer’s charm, humor, and storytelling abilities, he found his work to lack the moral and philosophical seriousness that Arnold deemed essential for poetry to be considered truly great or "classic."
Thus, Arnold praised Chaucer's talent but still concluded that Chaucer did not belong among the "great classics" due to this absence of high seriousness—an essential criterion in Arnold's critical framework.