Final answer:
The nurse should know that the client can add one tablespoon low-fat margarine to the oatmeal on his breakfast tray.
Step-by-step explanation:
The nurse should know that the client can add one tablespoon low-fat margarine to the oatmeal on his breakfast tray. This is because the client is on a 2,000-calorie ADA diet and has substituted whole milk with skim milk. Skim milk is lower in fat and calories compared to whole milk, so to maintain the calorie level of the meal, the client can add a small amount of low-fat margarine.