If a country has a very large working-age population, investment in which two areas together would be most useful?

Correct answer: Skills and employment

A large working-age population can become an economic opportunity only when people are both capable of productive work and able to find suitable jobs. Skills improve productivity, adaptability, and earnings, while employment provides the actual opportunity to use those skills and contribute to production, income, and public revenue. The two investments therefore support each other.

Training without enough jobs may leave skilled people unemployed or underemployed. Similarly, creating jobs without improving education and skills can leave vacancies unfilled or reduce productivity. Pensions and old-age homes serve important purposes but do not directly use the potential of a large working-age group. Primary schools and infant vaccination are valuable long-term investments, yet they do not address the immediate needs of workers. Thus A is the most useful combined choice.