Final answer:
Expecto Patronum is the spell used to create a protective Patronus against Dementors, involving happy memories to cast.
Step-by-step explanation:
The spell that provides protection against Dementors is Expecto Patronum. This spell is used to conjure a Patronus, which is a magical guardian, usually in the form of an animal, that acts as a shield between the caster and a Dementor. The Patronus represents something deeply positive and is created from happy memories the wizard or witch uses as a focus when casting Expecto Patronum, which is why it is among the most powerful defensive charms known to wizardkind.