The phrase that best supports the theme that "monstrous appearances can be deceiving" is D. harmless, and free.
This phrase describes in a simple way how being monstrous or hideous is not a condition that means you can't be actually harmless and free.
The important thing to understand in this specific phrase is the idea that your physical appearance cannot determine how and who you really are, and in this way we can understand that monstrous appearances can in fact be deceiving. In this specific case, the monster wants a companion to live a harmless and safe life, even though his physical appearance could socially mean he is as monstrous on the inside as he is on the outside.
The final conclusion would be: how someone looks can't show who a person is on the inside.
