We can choose the following option as the best way to use the given words to form a sentence:
C. Tim'll try his best to improve his Spanish pronunciation.
- All options initiate by talking about a future action. They all use "will" contracted with "Tim".
- Now, after the word "best", we cannot use a verb with -ing. The adjective "best" is followed by a verb preceded by "to". So we can already eliminate options A and D.
- There is no need whatsoever to use "on" after the verb "improve". Thus, we can eliminate option B.
- We can conclude that option C is the best one: Tim'll try his best to improve his Spanish pronunciation.
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