I'd say it's letter B - Unchecked emotions are potentially destructive.
The theme of this poem is the destructive potential of hatred and desire. Frost explores the two forces which have the potential to bring destruction to the world. Fire is desire, and ice is hate. The last line (“For destruction, ice is also great and would suffice”) suggests that it does not matter which is stronger. Both fire and ice are destructive. In the end, the moral idea that powerful emotions (passion, desire, greed, and hatred) have the capability to destroy the whole world no matter what kind of emotions they are.
