Concept:Efficiency of labour is improved by factors that directly raise workers' output and well-being.Explanation:A good work environment increases worker morale and reduces fatigue, leading to higher productivity.Good health status gives workers more energy and fewer absences, which also improves efficiency.Other options like school leaving age, population size, retirement age, or number of disabled or part-time workers do not directly increase labour efficiency.Answer:C. Work environment and health status of workers