C - The Arkenstone. Thorin mentions its status several times, and says he considers it to be the most important thing in Smaug's hoard.
As for the other answers ...
The map of Thrain is a symbol specifically of Thorin's family lineage (earlier ancestors to Thror to Thrain to Thorin -- the last step via Gandalf), not the dwarves in general. Thorin's family is important, because Thorin is the hereditary rightful King Under the Mountain, but they don't represent the entire dwarvish race.
The Valley of Dale is the former location of the human settlement next to the Lonely Mountain, and it was destroyed when Smaug arrived. It has nothing to do with the dwarves.
The Black Arrow is an heirloom of Bard's family, and also has nothing to do with the dwarves.