Incorrect spelling


Incorrect spelling, explanation: preferrably is incorrect, because no doubling of r is required since the last syllable is not the stressed one. Taking this into consideration preferably is the correct form. Preferably just like preference isn’t stressed on the last syllable, so r isn’t doubled.

Correct spelling


Correct spelling, explanation: although prefer ends with consonant-vowel-consonant combination, r isn’t doubled when a suffix able is added, because the last syllable of preferably isn’t stressed. Therefore the correct form is preferably, not preferrably.

Definition of preferably:
adverb, in a way that is preferable
The most wanted were lots located in quiet, preferably green areas.
I would like to work different hours in my new job, preferably 9 to 5.