For an essay question like this, I would consider both sides of the statement and make around three main points. The main focus of the statement is the concept of the tragic hero, so the student should consider the elements of a tragic hero (anagnorisis, harmartia, peripeteia) and decide the extent to which the characters experience them.
Personally, these would be my main points: (in the format of a plan)1) Disagree - Stanley cannot be a tragic hero as he lacks the typical elements of a classical tragic hero.2) Agree - Stanley is a tragic hero too, in the new sense of the word. He is a modern tragic hero and the play is a domestic tragedy. 3) Agree - There is not just one tragic hero in the play. Stella can be considered a tragic heroine too, as she falls from a high status to a low one. Overall conclusion: The concept of tragedy has changed with time, such that there can be multiple tragic heroes in a single work of literature. The reception of this play is dependent on the audience (modern audiences would sympathise with Blanche and consider her a tragic heroine, contemporary ones would not).