Explanation

For a client with COPD receiving supplemental oxygen, the most important aspect of the nurse's assessment is the oxygen saturation on pulse oximetry. The goal is to maintain the saturation at 88-92%. Saturations higher than this may indicate that the oxygen flow is too high, which can lead to complications like carbon dioxide retention.

While the presence of a productive cough, peripheral edema, and anxiety are all important to assess in a client with COPD, they do not directly relate to the safety and effectiveness of the oxygen therapy.