Final answer:
OBRA '90 requires pharmacies to offer Medicaid patients Medication therapy management service to optimize medication use and improve health outcomes.
Step-by-step explanation:
The service that OBRA '90 requires pharmacies to offer Medicaid patients is Medication therapy management. Medication therapy management (MTM) is a comprehensive service that involves pharmacists working directly with patients to optimize medication use and improve health outcomes. It includes activities such as reviewing medication profiles, identifying and resolving drug therapy problems, providing patient education, and collaborating with other healthcare providers.