Final answer:
The sentence in question is a complex sentence as it contains both an independent and a dependent clause, with 'Mayor Santiago' being the subject of the independent clause.
Step-by-step explanation:
The sentence 'Mayor Santiago started a successful program for young musicians after he left city politics' is not a simple sentence. It is a complex sentence because it contains an independent clause 'Mayor Santiago started a successful program for young musicians' and a dependent clause 'after he left city politics' which provides additional information about the time of the action. A simple sentence contains only one independent clause and no dependent clauses. In this instance, Mayor Santiago is the subject who performs the action, and the independent clause discusses his initiative for young musicians, demonstrating his continued impact on the community even after his political career.