Options include:
* Στεφανία (Stefania): This is probably the closest and most common way to write Stephanie in Greek, using a more common Greek spelling based on the root word.
* Στεφανί (Stefani): A shorter version, also quite common.
So, while there's no single "correct" answer, Στεφανία (Stefania) is the most widely used and accepted Greek equivalent of Stephanie.