Answer: C) But so you miss that modest charm / Which is the surest charm of all
Explanation:
The poem describes a very beautiful woman with a proud and distasteful character. The author is advising her to be more humble so that people may relate better with her.
In saying that the ''modest charm'' is the ''surest charm of all'', it can be implied that the author thinks that the most valuable and surest quality is modesty which means that it transcends even physical beauty.