User adoption and training are crucial for the successful implementation of any new system. Prioritizing end-to-end user training programs and support resources is essential to ensure that all users are comfortable and proficient with the new tools and functionalities. This approach helps to maximize user engagement and productivity, which are critical for the overall success of the project. Therefore, 'C. End-to-end user training programs and support resources' should be the focus in the implementation plan to enhance user adoption and effectiveness. Reference: Salesforce Training and Certification