Final answer:
When unable to get a blood return from a central venous access device, the nurse should first try to reposition the patient. This can potentially resolve the issue and confirm the device's patency before proceeding with chemotherapy administration.
Step-by-step explanation:
The nurse's initial action when unable to obtain a blood return from an unobstructed central venous access device should be B reposition the patient. This non-invasive maneuver can sometimes resolve the issue of no blood return. If repositioning does not work, the nurse may consider other actions such as checking for catheter patency, flushing the line, or consulting with a physician. Administering chemotherapy without ensuring patency and blood return can lead to serious complications.