What question could best be answered by observing food left out for 2 days?

Observing food left out for 2 days provides insight into how different foods interact with their environment, particularly in relation to attracting pests such as flies. The primary focus of this observation is on the attraction of flies, which is influenced by several factors including food composition, decay, and scent. By monitoring this, one can determine which types of food are more appealing to flies over a specified time period.

While nutritional value and shelf life pertain to the characteristics of the food itself, and decomposition involves the breakdown processes which might not be directly observable in a short timeframe, the behavior of flies in response to visible food waste is a clear, observable outcome. This makes the question about which food attracts the most flies the most appropriate focus for such observation, as the results can be directly measured and analyzed after the two-day period.