Final answer:
The verb 'ser' is used to express permanent characteristics, such as professions.
Step-by-step explanation:
Ellos son profesores y son muy inteligentes.
The verb ser is used to express characteristics that are inherent or permanent, such as professions. In this case, the sentence states that 'they are teachers' and this is a permanent characteristic of them. Therefore, we use ser.
Learn more about Use of ser and estar in Spanish