Final answer:
The Holy Mantle in The Binding of Isaac protects the player from the first instance of damage upon entering each room, thus allowing the player to safely enter and exit Curse Rooms.
Step-by-step explanation:
In the game The Binding of Isaac, the item that provides the ability to nullify damage the first time you are hit upon entering a new room is C) Holy Mantle. This item significantly enhances the player's survivability by allowing them to enter and exit Curse Rooms without taking any damage, as the Holy Mantle protects against the first instance of damage in each room. It is one of the many items that can critically alter gameplay and strategies in The Binding of Isaac.