Correct Answer: Ignore the past

Solution : The fourth option is the correct answer.

The idiom let bygones be bygones means to forgive or forget about past grievances or conflicts. In the context of the sentence, Nancy had been holding a grudge against her teacher, but she decided to let bygones be bygones, indicating her choice to move on and not dwell on past grievances. Ignore the past captures the essence of this idiom, emphasising the idea of not allowing past issues to affect the present or future.

Therefore, the correct answer is, ignore the past.