The correct answer is - It is an example of sophisticated mosaic art commonly found in Byzantine structures.
The Byzantines were very fond of artistic expression, and of those ways were the mosaics. They continued the tradition of the Romans of making mosaics, but as the time went on they managed to make them even more beautiful, with using new, more sophisticated techniques in making them. On this picture, we can see that a Christian saint is depicted, and since the Byzantine Empire was the seat and home of the Orthodox Christianity, this was very common motif in making the mosaics.