I think that the subject of the excerpt is c) Mathilde's discontent with her humble lifestyle because she compared her life to a rich one, one she felt that she was meant to have.
Short Answer C This has nothing at all to do with general social commentary. What commentary there is points to Mathilde's terminal discontent. (Terminal in the sense that nothing can cure it except living expensively with expensive things around her.) B and D are both not your best choice given these statements.
A is much harder to get rid of. It is not the love of expensive things that gets to her. It is what those expensive things mean. She recognizes that people judge her by her drab clothing and the humbleness of the surroundings she owns and lives in.
Others have put her in a social strata and she doesn't like it. C is your very best answer.
