Final answer:
The stem-changing conjugation of 'preferir' is 'prefieres'.
Step-by-step explanation:
The stem-changing conjugation of the verb 'preferir' is prefieres. In this form, the stem of the verb changes from 'e' to 'ie' in all the pronouns except for 'nosotros' and 'vosotros'. This means that the letter 'e' in the stem of 'preferir' changes to 'ie' when conjugated in the second-person singular form, which is 'tú'.
Learn more about Spanish verb conjugation