The question requires us to choose the correctly spelled word from the given options. Let's analyze each option:
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Sesquipedalian: This word is correctly spelled. It refers to a person who uses long words, or pertaining to long words.
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Sesquipadalian: This is incorrectly spelled. The correct form uses 'ped' instead of 'pad'.
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Sesquisepedalian: This is also incorrectly spelled. The error lies in the unnecessarily added 'se'.
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Sesquipedilian: This option is wrongly spelled due to the alteration of 'alian' to 'ilian'.
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Sesquipedaliain: This is an incorrect spelling due to the addition of an extra 'i'.
Thus, the correctly spelled word is Sesquipedalian.
Conclusion: The correct answer is Sesquipedalian as it matches the standard English spelling used to describe someone who uses long words.