HELP!! secret garden

Read the passage from the secret garden:

Then Mary knew Dickson meant the picture to be a message. He had meant that she might be sure he would keep her secret. Her garden was her nest and she was like a missel thrush. Oh, how she did like that queer, common boy.

Which theme is best developed by this passage?

A.) The secret garden was a place of healing and growing
B.) Secrets are dangerous
C.) The secret garden is a magical place.
D.) Happiness depends upon helping others

(K12 online middle school) Thanks!!