An investor wants to know how much he could have earned if he had put his money into a different investment. This is known as the ______, and it's also the minimum rate of return an investor requires.

The term that describes what the investor could have earned by choosing a different investment is opportunity cost. Opportunity cost represents the potential benefits that an individual misses out on when choosing one alternative over another. For an investor, it includes the minimum rate of return they could expect from an alternative investment, which serves as a benchmark for evaluating the attractiveness of any current investment opportunity.

The concept of opportunity cost is essential in finance because it reminds investors to consider what they are giving up when committing their resources to a particular option. This helps in making informed decisions about where to allocate capital in pursuit of the greatest potential returns.

In the context of the choices provided, while "cost of capital" refers to the cost of funds used for financing a business and can relate to the minimum expected return in some respects, it does not specifically capture the essence of foregone earnings from alternate investments. The other terms, such as net present value and return on investment, are more focused on measuring specific investment performances rather than emphasizing the comparative aspect of opportunity cost.