Answer:
Preferir is a stem changing verb as e changes to i in the present tense:
yo - prefiero
tu - prefieres
él/ella/Ud - prefiere
nos - preferimos
vos - preferís
ellos/ellas/ Uds - prefieren
Explanation:
Note: The change from e to i does not apply to vosotros or nosotros.
It also does not appear in other tenses except Preterite él because of pronunciation.
