The answer to this question is:Letter a.
When Viola and Sebastian see one another in Act V, they are overjoyed.
Twelfth Night Scene 5
>"Dazed, all the others look at Sebastian and Viola, who finally see one another. They interrogate one another with a barrage of questions about their birth and family history. Finally, they believe that they have each found their lost sibling. Viola excitedly tells Sebastian to wait until she has put her woman’s clothing back on—and everyone suddenly realizes that Cesario is really a woman."
>Viola and Sebastian see each other again, and there is a joyful reunion.
