Sensitivity labels in Microsoft 365 are used to classify and protect sensitive information within documents and emails. When applied to a Microsoft Word document, sensitivity labels can enforce specific protections and policies, such as applying watermarks to indicate the sensitivity level of the document. Applying a watermark is a visible indication that can inform users about the confidentiality of the document, helping to ensure that the content is treated appropriately. This is especially important for sensitive information, as it continuously reminds users of the document's classification, even when printed or shared. The other options, while they have their own security and usability functions, do not have the same direct link to sensitivity labeling. For instance, a password protects access to a document but does not indicate its sensitivity level. A digital signature verifies the authenticity and integrity of a document but is not related to its classification or how it should be handled based on its sensitivity. An index, while useful for navigation and organization within documents, does not impact the document's sensitivity or classification either. Thus, the most appropriate application of sensitivity labels in this context is through watermarks, enhancing document awareness regarding sensitivity levels.