In a scenario where the development team plans to enhance functionality of an existing application and wants to pilot the enhancements to a small group of users first, maintaining high reuse and maintainability is crucial. The most effective approach is:

D. Place the updated rules into a new minor version of the ruleset and include the new ruleset version in a new version of the application: This method leverages Pega's versioning and application structure to facilitate controlled access to new features. By creating a new minor version of the ruleset for the updated UI rules, the changes are neatly encapsulated, making them easy to manage and deploy. Including this new ruleset version in a new version of the application allows for specific user groups to be targeted for the pilot without disrupting the main user base. This approach maximizes reuse by building on existing rulesets and maintainability by clearly segregating enhancements for easy management and rollback if necessary.

Options A, B, and C do not offer the same level of granularity and control over the deployment and access to the enhanced functionalities, making D the most suitable choice for achieving the objectives of reuse and maintainability while piloting new features.