Answer: a) recovering from the flu.
Explanation: Positive energy balance is the balance of energy that is obtained when the energy intake is not altering many changes in already present body energy. It happens when intake of calorie is more than expenditure of energy.
Thus, the most appropriate situation for being positive energy balanced is during flu situation it is related with the infection with upper respiration system. So, energy intake should be proper so that no change can disturb the system.
Other option are not necessarily appropriate because exercise program and good health adult already have good environment . Stress does require balance of mind and energy.Thus, the correct option is option(a).