The question is not what it is; the riddle states it's a bone. No, the question is "WHICH" bone. It gives us choices of "bones" to find out which it is, so all answers before #7 are incorrect.
>>7
You were right in choosing a bone, but you fail to explain the part about the moon.
No, I think that the answer is associated with the shadow from an object lying on the ground. Because as the moon moves, the shadow gets longer, then shorter as the "blood" soaks into the ground. The "bone" could be any object, but the riddle tells us it's a bone.
Therefore, I believe that the answer is:
The bone that casts a shadow.