I believe that It is C, because although the passage does focus on Mathilde's love for expensive things, broadly the subject is how Mathilde is discontent with the lifestyle she is now living, and D is too broad, and B is also too broad.
You could show her how your feeling and tell her why
If trains were still continued to the future the first thing I would help them become more safe. I would do this by allowing them to stop faster and limiting the speeds. Hopefully this would help the society from getting injuries by the trains around them or even being in the train.
