To assist Celina, who struggled with repetitiveness in her vocabulary, a thesaurus would be the most beneficial reference source. A thesaurus provides synonyms and antonyms that can help her diversify her word choice and enhance the overall richness of her writing.
Mike, on the other hand, requires a dictionary to help him comprehend the correct meanings and usages of words, particularly in terms of parts of speech. By consulting a dictionary, he can clarify how to properly integrate more advanced vocabulary into his writing, ensuring that he uses words in the context that matches their grammatical function.
For Toni, who misused words with incorrect definitions, a dictionary would also be essential. However, it would be particularly important for her to utilize it not only for definitions but also for example sentences and phonetic spelling, which can help her see how words are appropriately used in context. Together, these reference sources will each cater to the specific writing challenges faced by Celina, Mike, and Toni, allowing them to improve their essays effectively.