The correct answer is D: "Two vast and trunkless legs of stone / Stand in the desert..."
The poem is actually discussing a great statue of the Egyptian pharoah Ramses II. The Greeks called Ramses by the name Ozymandias.
In the poem, a traveler tells the speaker about "two vast and trunkless legs of stone [that] stand in the desert..." This description refers to the statue of Ramses, or Ozymandias, which is now damaged and in pieces.