Hey there!
Here is your answer:
The proper answer to this question is option D "the locked door to the secret garden".
Reason:
The locked door represents the closed heart, to secret garden represents his heart. Doing questions like these its better to read the sentence slowly and use context clues in order to find the answer, for example: The locked door would better represent the closed heart because no one could get in. The secret garden would better represent the inside of the heart since no one knows what is in it, and has all your feelings, and emotions.
Therefore the answer is option D!
If you need anymore help feel free to ask me!
Hope this helps!
~Nonportrit