Which of the following would not be considered an act of investing in capital?

A) a father paying for his daughter's college education
B) a firm buying a new energy-saving machine
C) the state government building a new road
D) the Postal Service delivering the mail

Investing in capital involves spending money on goods or services that can improve future productivity or income. Choices A, B, and C all represent investments in human capital, physical capital, and infrastructure, respectively, which can lead to increased productivity or economic growth. Choice D, the Postal Service delivering the mail, is an operational expense rather than an investment in capital, as it does not directly increase the productive capacity of the economy.