Which of the following is true about external services?

The statement that much of the work to register an external service in Salesforce can be performed declaratively is accurate. Salesforce provides a user-friendly interface that allows administrators to set up external services without needing extensive programming knowledge. This declarative approach enables users to integrate external APIs directly into Salesforce by defining the service, registering it, and configuring it to work with Salesforce objects through a series of point-and-click steps.

Using this declarative method streamlines the process of connecting Salesforce with external systems, making it accessible for users who may not have a technical background. It allows for the creation of invocable processes that can interact with those external services seamlessly through the use of the metadata tools provided in Salesforce, further enhancing process automation capabilities.

While other options may touch upon aspects of external services, the requirement for programming skills or the optional nature of OpenAPI specifications detracts from the focus on the declarative capabilities highlighted in the correct answer. Additionally, the steps involved in using Named Credentials are specifically for managing authentication and access to external services rather than creating the services themselves.