Which keyword should be used in a Visualforce page to define object-level security?

The keyword used in a Visualforce page to define object-level security is "With Sharing." When this keyword is employed, the Visualforce controller enforces the sharing rules established within Salesforce. This means that if a user does not have access to certain records based on the sharing rules, those records will not be accessible through the Visualforce page.

By using "With Sharing," you ensure that your application respects the user's permissions and visibility settings, providing a secure user experience that adheres to Salesforce's data access policies. This is crucial for maintaining data integrity and privacy across different users and roles within the organization.

The other choices do not facilitate this kind of security. "Without Sharing" allows the controller to ignore sharing rules, potentially exposing sensitive information to users who shouldn’t have access. "No Sharing" is not a valid keyword in this context, and "Sharing" alone does not specifically define the behavior expected from the controller regarding user permissions. Therefore, "With Sharing" is the correct choice for implementing proper object-level security in a Visualforce page.