What forms do you stem change?

In Spanish, stem-changing verbs typically undergo changes in all forms except for the nosotros and vosotros forms. Therefore, if you are referring to the forms affected by stem changes, the correct answer would be:

**Yo, Tú, Ella, Él, Usted, Ellos, Ellas, and Ustedes**

This means that stem changes occur in the first person singular (yo), second person singular (tú), third person singular (ella, él, usted), and third person plural (ellos, ellas, ustedes) forms. The nosotros and vosotros forms generally do not undergo stem changes in the present tense.