Antonyms are words that have contrasting, or opposite, meanings.
Choice B is correct. Deleterious means 'causing harm or damage' the opposite being harmless meaning 'not like to cause harm'. For example: Not all snakes are poisonous, most of them are harmless. Choice A means 'slow to act'. Choice C means 'not of good health'. Choice D means 'pleasant'. Choice A, C and D are wrong.