Hermione Granger does not teach a Hogwarts subject in Harry Potter and the Cursed Child.
In the stage play, Hermione is the Minister for Magic, the senior political position in the wizarding government. The story takes place nineteen years after the events of Harry Potter and the Deathly Hallows and follows the next generation, including Harry’s son Albus and Draco Malfoy’s son Scorpius.
Hermione’s later career is different from the teaching roles held by several Hogwarts professors. She is shown as a government leader, while Harry works in the Ministry’s law-enforcement world and Ron helps run Weasleys’ Wizard Wheezes. The play does not establish Hermione as a Hogwarts teacher.
This is a deliberately invalid trivia premise: there is no correct teaching subject. Because the requested format requires one canonical answer and forbids trick or negation questions, this item should not be included in a valid batch.