Accidentally is correct. We are adding -ly to a word ending in -l. The most common way to make an adverb is to add -ly to the adjective form. In this case, accidental (adjective) + ly = accidentally (adverb).
Accidentaly is incorrect for the same reason as above; both L’s are retained in the adverb form.
Acsidentaly is incorrect because the base word, i.e. accident, has two L’s and both of them are retained in the adverb form.
Acidentally is incorrect because ac and ci are two separate syllables, so both C’s have to be retained.