A. It emphasizes the point that Xury is Crusoe's property and Crusoe can do with him as he pleases.
He uses the pronoun my to show ownership. During this time slaves were often called "boy". It was one more way they were seen as dependent upon their owners because they were not seen as people who could have the adult responsibilities to make them go from boy to man. While it is clear that Crusoe does value Xury, the phrase "my boy" is not highlighting a fondness that Crusoe has for him.