Read this excerpt from "Shut Not Your Doors" by Walt Whitman. What is the form of the poem?

"Shut not your doors to me, proud libraries,
For that which was lacking on all your well-fill'd shelves, yet needed most, I bring;
Forth from the army, the war emerging-a book I have made,
The words of my book nothing-the drift of it everything;
A book separate, not link'd with the rest, nor felt by the intellect,
But you, ye untold latencies, will thrill to every page;"

A. sestina
B. Shakespearean sonnet
C. haiku
D. free verse