She enjoyed herself immensely at the party
means, she had a good time at the party.
- She had a terrible time at the party - means her time was bad at the party.
- She had a horrible time at the party - same as above; her time was bad at the party.
- She had a terrific time at the party - means she had a good time at the party.
- She had a terrifying time at the party - means she had a frightening (not good) time at the party.
So, correct option: C.