Answer:
They must be the same part of speech.
Explanation:
A is wrong because not only nouns can have synonyms e.g. happy, excited
B is wrong because they do not have to sound the same e.g. food, vittles
C is correct because they have to mean the same and that would not happen if they were not the same part of speech
D is wrong because synonyms mean the same, antonyms mean different things. e.g. sad, disappointed
