Explanation:

Let’s break it down:

- Human Capital (Option 1): When people gain education, health, or skills, it doesn’t just help them personally (higher income, better job); it also helps society at large (more innovation, higher productivity, stronger communities). So, investing in people creates both public and private benefits.

- Physical Capital (Option 2): Factories, machines, and infrastructure definitely boost productivity and personal wealth. But while they have wider effects, the real public-private benefit combo is stronger with human capital.

- Both (a) and (b) (Option 3): You might think this makes sense, but the true textbook answer zeroes in on human capital for this specific dual benefit.

- None of the above (Option 4): Nope. There are clear right answers here.

Option 1: Human Capital is correct.