Final answer:
The nurse providing patient teaching to a chemotherapy patient is using the principle of autonomy, which upholds the patient's right to make informed decisions about their own health care.
Step-by-step explanation:
The ethical principle that the nurse is using when providing patient teaching for a patient undergoing chemotherapy is the principle of autonomy. This principle emphasizes that patients have the right to exercise agency or self-determination in making decisions about their own health care. By offering patient teaching, the nurse is supporting the patient's right to make informed decisions regarding their treatment options and intervening in a manner that respects their independence and personal choice in the matter.