Which is the most likely reason a writer would use a thesaurus? to find the pronunciation of a word the writer does not know how to say to find a synonym for a word that the writer has used repeatedly and wants to vary to find the history of a word in order to make associations with other words that share the origin

The correct answer is to find a synonym for a word that the writer has used repeatedly and wants to vary.

A thesaurus is a dictionary of synonyms - you would open up a thesaurus if you want to find words that have the same (or very similar) meanings as another word.

If you want to check pronunciation, you can look it up in a regular dictionary. If you want to see the history of a word, you would need an etymological dictionary.