Final answer:
The PTA should be behind the patient while they descend stairs with an assistive device, providing safety and support.
Step-by-step explanation:
The correct placement for the Physical Therapist Assistant (PTA) to be in when guarding a patient who is descending stairs with an assistive device is b. Behind. This position allows the PTA to act quickly and effectively in case the patient loses balance or needs additional support. Being behind the patient provides a safety buffer and enables the PTA to support the patient's trunk or assistive device if necessary, offering stability and confidence to the patient as they navigate the stairs.