Correct spelling, explanation: this verb has a long history. People used it in the 14th century and it was spelled preferren. In this form, we can see double -r, not double -f. We should write this word with one consonant. Thus, remember, that the only correct spelling is prefer.

Definition of prefer:
verb, to like better or best
I prefer when you talk to me in person.
Alex prefers his coffee hot.

Expressions and phrases with prefer:
Some commonly used expressions with prefer are: if you prefer, you can; would prefer an alternative; and would prefer (if) you didn’t [go, use].
If you prefer, you can go home without me.
Your father would prefer if you didn’t go to the party.

Incorrect spelling, explanation: when one’s curious about the spelling of a specific word, they should always look at the etymology. In this case, we can clearly see that the form preffer doesn’t exist and it never did. There’s no reason to add another -f letter. The only right spelling is: prefer.

Incorrect spelling

Incorrect spelling