What would a business use to provide a fixed price for their services that cannot be altered afterward?

The correct choice is a quote. A quote is a formal document presented to a customer that outlines the price for specific services or products. Once a quote is provided, it typically locks in that price for the customer, meaning it cannot be altered without agreement from both parties. This makes it a reliable basis for a contract or agreement since the customer can be confident about what they will pay for the services.

In contrast, an estimate is usually an initial approximation that may vary and is not binding. A bid often refers to competitive proposals that businesses provide during a tendering process and can also be subject to change. A proposal is a broader document that outlines the details of the services and may include pricing but is more about the overall pitch rather than just fixing the price. Hence, for establishing a fixed price that remains unchanged after being provided, a quote is the most appropriate choice.