In general, the skeleton is usually what becomes preserved in fossil. This is because over time, soft tissues and organs will degrade and decompose before the necessary time passes for fossilization. Bones are much more rigid and are able to overcome the process of fossilization. Based on this information, we can eliminate the skeleton and probably the feathers. We can eliminate feathers because we know and have evidence that feathers are capable of being fossilized. I would also say that we could eliminate the beak for this question. That's because birds' beaks are made of a protein called beta keratin, which is quite hard and durable. This leaves you with the decision of either the skin or internal organs. Which one do you think would degrade first? Think about the predators that may come by and eat parts of the bird after it dies.