The correct option is A Clean
Antonyms are words which have the opposite meaning in comparison to the other. Option B)clean is correct as the word clean means free from dirt, marks, or stainsand the word nasty means very bad or unpleasant.. The other options are wrong as sad, filthy, and challenging mean feeling or showing sorrow; unhappy, disgustingly dirty, and testing one's abilities; demandingrespectively. The correct answer is B)clean.