Final answer:
When energy input is less than energy output, the person is in negative energy balance, meaning that the body utilizes stored energy, leading to a decrease in internal energy and potential weight loss.
Step-by-step explanation:
When energy input is less than energy output, the person is in negative energy balance. This is because the internal energy will decrease as a consequence of the body burning stored energy to compensate for the lack of incoming energy through consumption. It means that energy storage, such as fats or carbohydrates, is being used to meet the energy requirements, which may lead to weight loss. This concept is related to the basic principles of energy homeostasis, which is the balance between the energy consumed in food and the energy expended in metabolism and physical activity.
To connect this with thermodynamics, according to the first law of thermodynamics, energy in an isolated system is conserved. However, when it comes to a living organism, if the energy output can be more than the energy input, this leads to a negative balance. So, the correct answer is option 2) 'Negative'. This implies that, while conversation laws are obeyed and thermodynamics is not violated, the organism must utilize its internal stores to supply the necessary energy.