Final answer:
Differences in human capital are produced by variations in the level, type, and quality of educational credentials, job training, and work experience.
Step-by-step explanation:
Human capital refers to the information, skills, knowledge, education, and health of workers that shape their ability to be productive. Differences in human capital are produced by variations in the level, type, and quality of educational credentials, job training, and work experience. For example, workers with higher levels of education and skills tend to have more human capital, which can lead to higher productivity and earning potential. Therefore, the correct answer is D) all of the above.