Answer:
A stem is the part of the verb that comes before the ending (which, in infinitive verbs, is always either ar, er, or ir). In regular conjugations, the stem doesn't change, where in irregular conjugations, it can be completely different. In stem-changing verbs, the change is in the last vowel of the stem.
Explanation:
Answer: B.) ni
Isabela: No puedes permitir entrar ni vendedores ni perros callejeros.
Isabela: You can not allow neither vendors nor stray dogs to enter.