Final answer:
The subject pronoun to address a professional in Spanish is 'usted'.
Step-by-step explanation:
The subject pronoun that you would use to address a professional in Spanish is usted. This pronoun is used to show respect and formality when addressing someone of higher social status or authority, such as a professional or an elderly person.
For example, in the given question, you would address Sr. Siqueiros using usted. So the correct sentence would be: ¿Es optimista o pesimista, usted, Sr. Siqueiros?
Learn more about Subject pronoun usage in Spanish