Synonyms are words which have the same or nearly the same meaning in the same language.
Antonyms are words which have opposite meaning in comparison to the other.
Option B)harmless is correct as the word harmless means not able or likely to cause harm and the word noxious means harmful, poisonous, or very unpleasant.
Both the words are opposite of each other.
The other options are not correct as they do not have the opposite meaning to the given word.
Notorious, shadow, and anxious means famous or well known, typically for some bad quality or deed,used in reference to proximity, ominous oppressiveness, or sadness and gloom,and feeling or showing worry, nervousness, or unease about something with an uncertain outcomerespectively.