Answer:
lemon juice
Step-by-step explanation:
Acids prevent browning because they react with the oxygen that comes into contact with the surface of the sample. Once all the acid (or whatever else is covering the surface) has reacted with the oxygen or the acid has degraded or washed off, then the sample will start to brown again. Stronger acids, like lemon juice, can even denature the enzyme. This means that the enzyme can no longer perform its original function because of its environment.