The Salesforce Lightning Design System (SLDS) is a CSS framework that provides a look and feel that is consistent with Lightning Experience1. By incorporating the SLDS CSS stylesheet into the JavaScript applications, the developer can apply the same styles and components that are used in Lightning Experience, without having to rewrite the entire application or change the underlying logic. This is the quickest and most effective way to update the custom applications to match the Lightning Experience UI.
The other options are not as quick or effective as option A. Option B would require rewriting all the Visualforce pages as Lightning components, which would be time-consuming and complex, especially if the applications use a third-party JavaScript framework that is not compatible with Lightning Web Components.
Option C is not valid, as there is no such attribute as enableLightning in the Visualforce page definition.
Option D would enable the Visualforce pages to be accessed from Lightning Experience, Lightning Communities, and the mobile app, but it would not change the appearance or behavior of the pages to match the Lightning Experience UI. References:
* Style with Lightning Design System - Salesforce Developers
* Using the Salesforce Lightning Design System in Apps - Salesforce Developers