The verb form that has changed its stem is "quieren."
In the present tense conjugation:
- "querer" (to want) undergoes a stem change in the forms "quiero," "quieres," "quiere," and "quieren," where the 'e' in the stem changes to 'ie.'
So, among the options given, "quieren" reflects the stem change from "quer-" to "quier-."