Which of the following are elements of a valid schema for External Services?

The correct choice emphasizes that a valid schema for External Services must adhere to the OpenAPI specification, with clearly named parameters.

This is important because the OpenAPI specification provides a standardized way to describe RESTful APIs, ensuring consistency and interoperability. By following this specification, Salesforce can understand and properly interpret the API being referenced, allowing for seamless integration. Named parameters further enhance clarity by making the data exchanged explicit, making it easier for developers and systems to work with the API's inputs and outputs.

The requirement for named parameters also aligns with best practices in API development, which promotes readability and usability. By ensuring parameters are well-defined, this reduces the likelihood of errors and enhances the documentation of the API.

Regarding the other options, they either fail to mention the OpenAPI specification, which is crucial for defining a valid schema, or they include elements that deviate from standard practices. For example, while complex objects and the specifics of character limits are relevant to some contexts, they do not encompass the fundamental requirements for schema validation in External Services as dictated by the OpenAPI framework.