How is CTA bus service primarily funded?

B is the correct choice because the funding for the Chicago Transit Authority's bus service primarily comes from multiple sources: passenger fares, local and state taxes, and government grants. Fare payments collected from riders constitute a significant portion of the revenue, while taxes, such as sales taxes and property taxes, provide essential funding as well. Additionally, government grants play a crucial role in supporting operational costs and infrastructure improvements.

In contrast, the other options do not play significant roles in the overall funding structure for CTA bus services. Advertising revenue, while it can contribute to income, is not the primary source of funding. Membership fees are not applicable to public transit systems like the CTA, as the service is open to all residents and visitors without requiring membership. Private donations are typically not a substantial or reliable source of revenue for a government-operated transit system like the CTA, as they depend heavily on public funding mechanisms.