The first sentence in this paragraph functions as an opening, an introduction to what the writer wishes to convey. He/she means to present a certain situation that, in his/her opinion, is a problem that demands attention. To present that point of view without directly using the word "problem", the writer chooses to refer to it as "a practice that should be avoided" in the first sentence. The audience now knows the writer's viewpoint: that what is about to be presented is a negative situation.
The passage states how the birthmark on Georgiana's cheek causes her to be "countenance even hideous" therefore the answer is B). the birthmark on Georgiana's cheek.
