Final answer:
The subject of this question is English and the correct answer is 'Let bygones be bygones.'
Step-by-step explanation:
The subject of this question is English since it deals with idiomatic expressions and the lesson to be learned from them.
The saying 'What's done is done. What's gone is gone. One of life's lessons is always moving on.' can be interpreted as the need to let go of the past and move forward. This is similar to the idiom 'Let bygones be bygones' which means to forget past grievances and forgive others. It is important to learn from the past and use it to grow and improve, as indicated by the options 'Yesterday is history' and 'Learn from your past'.
The correct answer in this context would be option a) Let bygones be bygones.