The Salesforce Lightning Design System (SLDS) is a CSS framework that provides the styles and design patterns to create user interfaces that match the Salesforce Lightning Experience. Since the existing applications are built with a third-party JavaScript framework, the most direct and efficient method to update their appearance is to incorporate the SLDS stylesheet. This allows developers to apply SLDS CSS classes to the existing HTML markup, achieving the desired look and feel without rewriting the application's core logic. This approach is significantly faster and more effective than a complete rewrite.

1. Salesforce Lightning Design System Documentation

Overview: "The Salesforce Lightning Design System (SLDS) is a CSS framework that allows you to build custom applications with the look and feel of Salesforce Lightning Experience without writing a single line of CSS." This confirms SLDS is the correct tool for styling.

2. Visualforce Developer Guide

Using the Lightning Design System in Visualforce Pages: The guide explicitly states

"To give your Visualforce pages the Lightning Experience look and feel

you can incorporate the Salesforce Lightning Design System (SLDS)." It details how to include SLDS using the tag or as a static resource

which is the action described in the correct answer. (See section: "Using the Lightning Design System in Visualforce Pages").

3. Visualforce Developer Guide

Component Reference: The documentation for the tag describes the lightningStylesheets attribute

confirming it is the correct attribute name and explaining its function

which is to apply Lightning styles to standard Visualforce components. This clarifies why option B is incorrect. (See section: "Component Reference > ").