Final answer:
The action by the nurse that best represents the evaluative portion of the nursing process is reassessing the client's pain level after administering pain medication.
Step-by-step explanation:
The action by the nurse that best represents the evaluative portion of the nursing process is D. Reassessing the client's pain level after administering pain medication. During the evaluation phase of the nursing process, the nurse assesses the effectiveness of the interventions provided and determines if the desired outcomes have been achieved. In this case, the nurse is reassessing the client's pain level to evaluate the effectiveness of the administered pain medication.