"emerging adults who manage their sleep patterns, monitor their food intake, and maintain a health weight are using skills to promote their own health."

The answer would be: self-control

A problem in sleep pattern is linked to several diseases because it could disrupt the secretion of hormones. Monitoring food intake will help to keep the body in the ideal range, preventing the disease from malnutrition.
The strategy that used by the adults in this case is simply controlling himself.