C. every stem inside the boot changes, all outside the boot stay the same
In the context of Spanish, stem-changing verbs are often referred to as "boot verbs" because the pattern of vowel changes occurs within the "boot" shape created when you visualize the conjugation forms of the verb (the singular forms and the third-person plural form) on a conjugation chart. The vowels in the "boot" (the forms that change) are the ones that undergo changes, while the forms outside the boot retain the original stem.