I think the answer is option C. Mark & Molly have been boyfriend and girlfriend. It's the only appropriate answer seems to me.
Lets review all the choices for the answer.
A. Molly likes Rob Becker because he plays basketball.
- In the paragraph Mark saw Molly talking on her cell phone after a basketball game. If Rob Becker were playing basketball, Molly would've talked to him face to face after the game.
B. Molly's parents have given her a strict curfew.
- If Molly's parents had given curfew, Molly wouldn't have gone to Linda's party in the first place.
C. Mark & Molly have been boyfriend and girlfriend.
- Mark had sinking feelings in his stomach after seeing Molly's holding hand with Rob Becker. This is the only option that connects correctly with this paragraph.
D. The voicemail on Molly's cell phone isn't working.
- Molly was able to receive voice mail. She was just avoiding Mark for some reasons. Cause when Mark saw Molly talking on her cell phone he called her. But she didn't pick up the call. That proves she knew Mark was calling her for some times.
I might be wrong but those are my analysis on the question & based on that I chose the answer.