Which action is prohibited regarding public workbooks created by others?

The correct answer is making modifications. When dealing with public workbooks created by others, users are typically allowed to view the content but do not have permission to alter the original data or layout of the workbook. This restriction is crucial for maintaining the integrity of the shared work, as it ensures that the original creator's intentions and designs remain intact.

Editing properties or adding comments might be permissible as these actions often do not alter the original content but rather provide additional context or insights. Exporting data could also be an action allowed under certain circumstances, as it does not require modifying the workbook directly. However, making modifications signifies a fundamental change to the workbook's core functionality or content, which is usually prohibited to safeguard the creator's work and maintain consistency across public resources.