Final answer:
The question seems to refer to literary character's perception of a mark, but the provided text discusses Aylmer's thoughts about a birthmark on Georgiana's cheek, not Felix's thoughts about a mark on Nic's back.
Step-by-step explanation:
The question seems to refer to an understanding of a character's perception of a mark in a literary piece. However, the provided excerpts do not clearly answer what Felix thinks the mark on Nic's back is, as Felix is not mentioned in the given context. Instead, the passages discuss a character named Aylmer and his thoughts about a birthmark on his wife Georgiana's cheek. Aylmer finds the birthmark, which is a crimson hand, to be a fatal flaw on his wife's beauty and is obsessed with its removal, despite the potential risks involved. The discussion of the mark's possible removal and its significance serves as an exploration of themes such as human perfection, control, and the consequences of scientific experimentation.