A systems designer needs to set up a protocol to facilitate secure authorization and access to resources within a user profile between sites without sharing passwords. Which protocol should the systems designer use?

The correct choice is OAuth, as it is specifically designed to grant access to resources on behalf of a user without the user having to share their passwords with multiple applications or services. OAuth is an open standard for access delegation commonly used as a way to provide secure delegated access, often used for token-based authentication.

OAuth works by allowing an application to obtain limited access to user accounts on an HTTP service, such as Facebook or Google, without exposing user passwords. The user authenticates with a service (the identity provider) and gives permission for a third-party application to access their information. The third-party app receives an access token, which it can use to interact with the service on the user’s behalf, establishing a secure authorization framework that enhances security and user experience.

This approach contrasts with protocols like LDAPS, RADIUS, and Diameter, which focus on directory access or network authentication. While these protocols add layers of security, they don't provide the same level of authorization to external applications without password sharing that OAuth does. Thus, OAuth is the most appropriate choice for facilitating secure authorization and access between sites while ensuring that passwords remain confidential.