I believe the answer is heroic couplets, which is a traditional form for English poetry, commonly used in epic and narrative poetry, and consisting of a rhyming pair of lines in iambic pentameter.
Blanche, by retreating into hysteria and madness, and by refusing to acknowledge her sister as she leaves the apartment with the doctor, may be sparing Stella the horror of having to face the truth about her husband. Blanche's descent into madness shields Stella from the truth.
I think its B because I remember my teacher talking about stanzas and their lines when going over enjabment!