Incorrect spelling, explanation: this is a mistaken form because there is no reason to double f in prefer. Preffer is also historically incorrect, looking at Latin (praeferre) and Old French (preferer) forms. Thus the correct form is prefer.

Correct spelling, explanation: prefer is the correct spelling because the word originates in Latin and later Old French, and in all of these forms, as well as 14th-century form, there was only one f (praeferre, preferer, and preferren). Therefore double f form is incorrect and the correct form is prefer.

Definition of prefer:
verb, to choose one thing over another, to select because of preferences
My wife prefers red wine, thank you.
Can we not go to the cinema today? I think I prefer to stay at home.

Incorrect spelling

Incorrect spelling