Explanation:

- Option 1: Sources

- Refers to the origins or ways of adding to the stock of human capital.

- Activities like education, training, and health care can increase human capital.

- Option 2: Determinants of physical capital

- Refers specifically to factors that affect the accumulation of physical assets, not human capital.

- Examples include machinery, infrastructure, and technology.

- Option 3: Both a and b

- Implies that both options 1 and 2 together describe the way to add to the stock of human capital.

- This is incorrect as physical capital determinants don't apply to human capital.

- Option 4: None of these

- Applicable if none of the options relate to the stock of human capital.

Correct Answer: Option 1: Sources