The final stanza of "Dover Beach" at first appears to be positive by the appearance of words like "love," "dreams," and "beautiful." However, as the stanza continues, it becomes clear that the author does not have a positive outlook on the world. In fact, Matthew Arnold's opinion on the world is actually quite negative.
The stanza begins with the speaker saying that he and his lover should be true to one another. The speaker says that the world before them "seems" beautiful -- of course, appearances are often different from reality. He goes on to say the "beautiful" world before them is "like a land of dreams." Arnold is making the point that what is beautiful and new in the world is really just an illusion.
Instead, the speaker says, the world does not have joy, love, or light. Arnold has a dark vision of the world, indeed. Arnold goes on to say that everyone in this world exists "on a darkling plain" -- basically a dark pasture. Darkness is often equated with a lack of knowledge (for example, the Dark Ages). This land is full of "struggle and flight." Even worse, "ignorant armies" constantly do battle there.
By the end of the stanza, it is clear that Arnold sees the world as a dark place full of confusion, turmoil, and disorder. As a result, only option D. ("The world is a confusing and hostile place.") best characterizes Arnold's view of the world.