Final answer:
The correct verb form is 'debe'.
Step-by-step explanation:
The correct verb form to complete the sentence is A. debe.
In Spanish, the verb 'deber' means 'to have to' or 'must.' In this sentence, the subject 'Mi hermana' is third person singular, so we use the form 'debe' to match the subject.
For example, 'Mi hermana debe cocinar un pastel de chocolate' translates to 'My sister must cook a chocolate cake.'
Learn more about Spanish verb conjugation