1) Education: Education is a primary source of human capital, as it equips individuals with knowledge and skills, enhancing their productivity and economic contribution.

2) Wealth: While wealth can support human capital development, it is not human capital itself. It can provide access to education and training, but it is not the knowledge and skills individuals possess.

3) Migration: Migration is a factor that can influence human capital, as it can lead to the movement of skilled individuals across regions, potentially impacting the human capital of both source and destination areas.

4) On-the-Job Training: On-the-job training contributes to human capital development by improving an individual's skills and knowledge in a specific work context, enhancing their value to employers and society.

Education is considered one of the main sources of human capital. Education contribute significantly to the development of an individual's skills, knowledge, and capabilities, which are essential components of human capital. Education enhances a person's ability to participate in the workforce and contribute to economic and social development.

Thus, The Answer is Education.