The spell that conjures a flock of birds is Avis. It is a conjuration charm in the Harry Potter universe, producing a group of birds from magical energy rather than summoning birds from a distant location.
The incantation comes from the Latin word avis, meaning “bird,” a pattern Rowling frequently uses when naming spells. Avis is notably demonstrated by Mr Ollivander during the wand-weighing ceremony in Harry Potter and the Goblet of Fire, when the conjured birds fly around the room.
Avis is easy to confuse with Oppugno. Avis creates or conjures the birds; Oppugno is the charm used to direct conjured creatures or objects to attack a target. The two spells can therefore appear together in explanations of the scene involving Hermione and the birds.
The other options are defensive, illumination or summoning spells: Expelliarmus disarms an opponent, Lumos creates wand-light and Accio summons an object. None of them specifically produces a flock of birds.