Letter D. Newgate prison. Wemmick works for Mr. Jaggers (a lawyer who defends criminals). So it makes sense to go to prison and "cultivate" the prisoners so they can hire him again as a lawyer when is needed, that is why he treats prison like a greenhouse (a place where things grow). It's a metaphor.
If it does not contain Carbon.
Answer:
I believe it is the line "till bore an apple bright" becusse it link to the story of Adam and Eve with the forbidden apple tree and apple also links to a poison treee