Final answer:
To prevent aspirin toxicity, the nurse should assess the serum potassium level.
Step-by-step explanation:
To help prevent aspirin toxicity in a client with a history of heart disease who has been prescribed prophylactic aspirin daily, the nurse should assess the serum potassium level (option D). Aspirin can cause renal toxicity and decrease serum potassium levels. Monitoring the serum potassium level will help detect any potential electrolyte imbalance and prevent complications.