Answer: A - How much the speaker loves the tree
Explanation:
The speaker is comparing in this stanza the importance of his heart to his life and the tree's bark and it's life he does this by stating that his heart (Or life) is as close to the tree as the tree's own bark. Meaning that it could not be D or B; The reason it is not C is because of it not showing how determined he is to save the tree, only showing that he dearly loves the tree. The only other option is A.