Answer:
C) it established a highly skilled professional army built around elite units
Step-by-step explanation:
The Immortals or Meloforos (in ancient Greek, hoi mĂȘlophoroi, literally "apple carriers"), was a Persian elite regiment of the royal guard who fought in the Greco-Persian Wars. The name Immortals comes from Herodotus who called them the Ten Thousand or Athanatoi (literally, "immortals"). The Persians themselves probably did not use this term, which may be a corruption of the word anusiya (companions). The term also referred to their strategy in which if one soldier fell, another replaced him making the illusion that they could not die.
They had the nickname of the "apple holder", because the base of the spear they carried was shaped like that fruit.
Herodotus mentions that the Immortals were a heavy infantry troop commanded by Hidarnes that always kept the number of 10,000 men: each dead, wounded or seriously ill member was immediately replaced by another, which is why they apparently never died. He accepted Persian and Median members.