A synonym is a word that's

A. similar in meaning.

B. pronounced the same.

C. opposite in meaning.

D. defined in a thesaurus.

A synonym is a word that is similar in meaning - the correct answer is A. "speak" and "talk" are synonyms.

The other answers can be confusing, I will explain why:

B: pronounced the same : this is a homonym. For example, "write" and "right" are homonyms.

C: these are antonyms: for example big and small are antonyms.

And as far as D is concerned: all words should be defined in Thesaurus!