Final answer:
Purchasing legumes and whole grains is the best option for buying foods that require low energy input, since they are more energy-efficient compared to animal products and help lower the dietary energy factor.
Step-by-step explanation:
Among the options provided, e. Buying legumes and whole grains is a way to purchase foods that require a low energy input. Legumes and whole grains are considered more energy-efficient because they generally require smaller amounts of energy to produce in comparison to meat and dairy products. The energy ratio for plant-based foods is lower than that of animal products, which means less energy is invested in their production for the amount of metabolic energy they provide.
Meat production, especially beef, requires a high energy input mainly due to the energy lost when converting plant calories into animal calories. This is reflected in the net energy gain concept that shows how foods like beef are energy losers because it takes far more calories of input to produce 100 kcal of meat than it does for plant-based foods such as rice.
Moreover, consuming non-meat and non-dairy foods aligns with promoting a sustainable diet with less wasted energy, which has a lower impact on the environment. Therefore, choosing plant proteins like legumes and whole grains helps in lowering the overall dietary energy factor and is a more energy-efficient food choice.