Synonyms are words that are

A. similar in meaning.

B. pronounced the same.

C. defined in a thesaurus.

D. opposite in meaning.

Synonyms are words that are similar in meaning. While synonyms can be found in a thesaurus, a thesaurus also shows antonyms, or words that have opposite meanings (which is also answer D), so A is the better answer. Answer B describes a homophone which are two words that sound the same, but have difference meanings.