The mistake is in part C. It should be "she's" instead of "she doesn't".
Corrected sentence: The reason she doesn't have many friends is that she's rude and unpleasant.

The mistake is in part B. It should be "because" instead of "because of".
Corrected sentence: I want to be ready because they may need a new goalkeeper.

The mistake is in part C. The word "so" should be removed.
Corrected sentence: Now that I had spent all my money, I couldn't go to the concert.