Which of the following actions can be performed without additional configuration in Salesforce?

Posting to Chatter can be performed without additional configuration in Salesforce because Chatter is built into the platform as a social collaboration tool intended for real-time communication within organizations. Users can post updates, share files, and collaborate without needing to configure additional settings.

In contrast, developing an Apex trigger requires knowledge of Apex code and the declarative capabilities of Salesforce, along with proper coding practices, which is inherently more complex and does need some level of configuration or setup in the development environment. Creating a Workflow Rule necessitates defining specific criteria and actions, thus necessitating configuration to ensure it functions as intended. Setting Field Level Security involves permissions that must be configured for user profiles, which is another layer of setup beyond default options.

Thus, posting to Chatter stands out as the action that can be executed directly and instantly within the platform.