None of the provided options correctly defines a "stem-changing verb" in Spanish. A stem-changing verb is one in which the stem (the part of the verb before the ending) undergoes a change when it is conjugated in certain forms.
In Spanish, stem-changing verbs can be categorized based on the type of change that occurs: e-ie, o-ue, e-i, or sometimes u-ue. These changes take place in the present tense and other tenses/moods for specific conjugations, typically affecting all but the nosotros and vosotros forms in the present tense.
For example, in the verb "pensar" (to think), the stem change is e-ie:
- Yo pienso (I think)
- Tú piensas (You think)
- Él/ella/Ud. piensa (He/she/You formal think)
- Nosotros pensamos (We think) - no stem change
- Vosotros pensáis (You all think) - no stem change
- Ellos/ellas/Uds. piensan (They/You all formal think)
Therefore, a correct definition of a "stem-changing verb" in Spanish would be:
a verb that has a change in the stem (the part of the verb before the ending) in certain forms when it is conjugated.