Correct spelling, explanation: this form is the correct one because all previous forms (Old French preferer, Latin praeferre, and later 14th-century preferren) were written with just one f. There is no grammatical or pronunciation reason to double the f in prefer. Therefore preffer is a misspelling, and prefer is the correct form.

Definition of prefer:
verb, to favour, to choose sth over sth else
I prefer cake to ice cream.
Do you prefer a view of the sea or the forest?

Incorrect spelling, explanation: preffer is an incorrect spelling because there is no need to double consonant f in prefer. Origins show that double f has never been there (for example in Old French preferer), so prefer is the correct form.

Incorrect spelling

Incorrect spelling