The correct answer is: A. ugly
An antonym is a word that means the opposite of another word. In this case, the word “beautiful” means something that is pleasing to the eye or mind, while the word “ugly” means something that is unpleasant to the eye or mind. Therefore, the antonym of “beautiful” is “ugly.”
The other options are not antonyms of “beautiful.” The word “dark” can mean something that is not light, but it can also mean something that is mysterious or romantic. The word “rough” can mean something that is not smooth, but it can also mean something that is tough or strong. The word “dirty” can mean something that is covered in dirt or filth, but it can also mean something that is immoral or corrupt.
Therefore, the only option that is an antonym of “beautiful” is “ugly.”