Answer:
The answer is False.
Step-by-step explanation:
Subject-verb agreement means the subject and verb must agree in number. This means that both the subject and the verb should be both singular or both plural. The subject cannot be singular and the verb plural and vice versa.
The above text shows a subject and verb not in agreement. The subject (one of) regardless of the words in between (...their grandmothers, their mother's mother) is singular.
This means that the verb too should be singular. In the above text, it is plural (are). The correct verb form is "is".
Therefore it should be: One of their grandmothers, their mother's mother, is a professor of music at the university.