Final answer:
Lilly should clear the cache of her web browser to ensure she has the most recent stock market information, and then refresh the browser.
Step-by-step explanation:
When Lilly notices that the stock market data on her screen has not changed for several hours, the most appropriate action she can take to ensure she has the most recent information is to clear the cache of her web browser. Clearing the cache will remove temporarily stored data that might be causing the information to appear stagnant. This can often resolve issues with not seeing updated content on websites. She should refresh her browser after clearing the cache to see if the stock market data updates.