Viola, at this point in the plot, is still under the belief that her only brother Sebastian has died of drowning during their shipwreck. She is still in disguise as a young man and her reply is a figurative and adorned way to say that he/she is the only surviving scion of his/her family.
Transcendentalism is the answer :)
hope thz help!
I think it’s the first one cause I read the story and it was actually pretty good