"I saw the Sultan's infantry drawn up in military array. They stood in silence so profound that I seemed to be looking upon statues... It is by merit that men rise in the service — a system that makes the Turk so formidable, and shames our own Christian mercenaries, who are unruly, mutinous, and ruled by birth rather than talent."

— Ogier Ghiselin de Busbecq, Flemish ambassador to the Ottoman court, Turkish Letters, 1555

Question: The system described by Busbecq MOST directly refers to which of the following Ottoman institutions?