What is the meaning of these idioms: stuck between a rock and a hard place and achille's heel.

Which sentence has a pronoun in the nominative case? a) we aren't sure if the group is going white-water rafting or not. b) dad refused to give the keys to her because of her disrespectful attitude. c) the radar could not see it because the plane had been covered with radar-resistant paint. d) diana wanted to go out with them, but her mother was resistant because of their reputation.