By mistake, unintentionally, inadvertently:
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accidentally
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accidently
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accidentely
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accidentily
A
Correct answer
Explanation
The correct spelling is 'accidentally' with the full '-ally' suffix. The adjective is 'accidental' (with 'al'), and to form the adverb, you add '-ly', resulting in '-ally'. The common misspelling 'accidently' drops one letter, which is incorrect.