The best solutions for minimizing potential downtime for users in this situation are to use named credentials and an enterprise ESB. Named credentials allow you to store the URLand authentication parameters of an external service in one definition, and then use it in Apex callouts or declarative integrations. If the URL of the external service changes, you only need to update the named credential once, and all the integrations that use it will be updated automatically. An enterprise ESB is a middleware layer that acts as a broker between different systems and services. It can provide routing, transformation, orchestration, and monitoring capabilities for integrations. If the URL of an external service changes, you only need to update the ESB configuration once, and all the integrations that use it will be updated automatically. Option B is not correct because remote site settings only allow you to specify the domains that are allowed for callouts from your org. They do not store any authentication parameters or provide any routing or transformation capabilities. Option C is not correct because content security policies are used to control what resources can be loaded on a Visualforce or Lightning page. They do not store any authentication parameters or provide any routing or transformation capabilities. References:

NamedCredentials

Streamline and Secure: Mastering Named Credentials in Salesforce