Answer: set the attribute enabling lighting to true in the definition.
Step-by-step explanation:
Since the company wants to update the applications to apply styling which resembles the look and feel of Lightning Experience, then the developer should set the attribute enabling lighting to true in the definition.
Based on the options given, it should be noted that rewriting all Visualforce pages as Lightning components, or incorporating the Salesforce Lightning Design System won't be able to achieve the goal of the company.