B. I thought I saw a dog bury their bone under the tree.
Explanation:
The pronoun their is incorrect in this sentence. Their is used to describe ownership of something that belongs to a human. For example: The twins blew out the candle on their birthday cake. But, it does not describe ownership of something that belongs to an animal like a dog. The proper sentence would be (and the sentence could use his or her, but I will use his for this example): I thought I saw a dog bury his bone under the tree.
