Which activity seems to be the most important health habit in old age?

The correct answer is calorie restriction.

Energy restriction or calorie restriction refers to a dietary routine, which minimizes the intake of calorie without undergoing malnutrition or a reduction in essential nutrients. The generally consumed components of food comprising calories are proteins, carbohydrates, and fats.

In various species, calorie restriction in the absence of malnutrition may slow down the process of biological aging, leading to the longer sustenance of health and an enhancement in both maximum and median lifespan.