Antonyms are words that have contrasting, or opposite, meanings.
Choice B is correct. Heretical means 'unorthodox' the opposite being orthodox meaning 'following religious practices strictly'. For example: The girl comes from an orthodox family. Choice A means 'devoutly religious. Choice C means 'a person who believes in mythical stories'. Choice D means 'ideas that are not in current practice'. Choice A, C and D are wrong.