What two behavioral factors are required for weight gain in the form of lean mass?

The correct answer emphasizes the critical combination of a positive caloric balance and high-volume, vigorous resistance training as key behavioral factors for gaining weight in the form of lean mass.

A positive caloric balance means that an individual is consuming more calories than they are expending. This surplus provides the necessary energy needed for the body to build and repair muscle tissue, especially when coupled with resistance training. During high-volume, vigorous resistance training, the muscles undergo stress and microscopic damage, which signals the body to adapt and grow stronger and larger. Therefore, this combination of extra energy (calories) and stimulus from resistance training is fundamental to promoting muscle hypertrophy and lean mass gain.

The other options do not support the objective of increasing lean mass. A negative caloric balance combined with moderate-intensity cardio would likely lead to weight loss rather than gain. Low-protein intake contradicts the need for adequate protein to support muscle repair and growth. Lastly, caloric restriction and endurance training typically contribute to fat loss rather than increasing lean mass, as endurance training often emphasizes energy expenditure rather than muscle building.