With stem-changing verbs, the stem often does not change for
a. first and second person singular
b. first and second person plural
c second and third person singular
d second and third person plural

Assuming this question is in Spanish, and not English, the correct answer is B. first and second person plural.
Take a stem changing verb in Spanish, such as decir (= to say). This is how it's conjugated: yo digo, tu dices, el/ella dice, nosotros decimos, vosotros decis, dicen. 
As you can see, nosotros and vosotros (first and second person plural) are the only forms whose stem doesn't change, whereas that change occurs in all other persons.