Which client would benefit most from immediate insulin administration?

Immediate insulin administration is most critical for a diabetic patient with a blood glucose level of 400 mg/dL. This level indicates significant hyperglycemia, which can lead to serious complications such as diabetic ketoacidosis (DKA) or hyperglycemic hyperosmolar state (HHS) if not addressed promptly. In cases of extreme high blood sugar, the patient's safety is at substantial risk, and administering insulin can help to quickly lower the blood glucose levels, stabilizing the patient’s condition.

In contrast, the other options do not present an urgent need for immediate insulin. A patient requiring an oral hypoglycemic for high fasting levels may be effectively managed with oral medications and does not necessitate the urgent intervention required for very high blood glucose levels. A post-operative patient with normal glucose levels would not need insulin since their blood sugar is already within a healthy range. Meanwhile, a capillary blood sugar reading of 250 mg/dL, while elevated, does not indicate the same level of critical urgency that a 400 mg/dL reading does, and could potentially be managed with diet, exercise, or oral agents rather than immediate insulin administration.