Reliability, maintainability, and MTBF describe how robust and serviceable rehab devices are during patient use. Reliability is the probability that the device will function as intended for a defined period under the expected use conditions. In rehab settings, that means during typical therapy sessions, under normal loads, power, and environmental factors. This matters because patient safety and therapy effectiveness depend on equipment performing reliably, minimizing interruptions and risk. Maintainability is about how easily and quickly a device can be serviced and restored to operation after a fault. It includes how easy diagnostics are, how modular the design is, and how readily spare parts and skilled service are available. In rehab devices, good maintainability reduces downtime, keeps therapy on track, and lowers maintenance costs. MTBF, or mean time between failures, is the average amount of operating time between failures under normal use. It helps plan preventive maintenance and predict downtime, so coverage and scheduling for repairs can be arranged to keep devices ready when needed. The statement that best fits these definitions says reliability is the probability of functioning under expected use; maintainability is ease of service; MTBF is mean time between failures. This aligns with how rehab devices are used and sustained in the field. The other options either misstate reliability or MTBF, or only partially capture maintainability without addressing the full concept.