Final answer:
Engaging involved staff at all levels in the decision-making process is the best approach to assess and adapt a medical unit to better meet the unique needs of older adult patients.
Step-by-step explanation:
The BEST approach to assess and adapt a 20-bed medical unit to better meet the unique needs of older adult patients, using complexity principles, would be to engage involved staff at all levels in the decision-making process. This approach recognizes that the staff members on the unit have valuable insights and perspectives that can contribute to the development of effective solutions. By involving staff members in the assessment and planning process, the nurse manager can tap into a wealth of knowledge and expertise.