The correct answer is b. the purchase of capital equipment and structures.
In economics, investment typically refers to spending on capital goods that will be used to produce goods and services in the future, such as machinery, buildings, and equipment. While purchasing stocks and bonds (option a) involves financial investment, it is not typically classified as "investment" in the traditional economic sense used in national accounts. Placing savings in a bank (option c) is related to saving rather than investment. The purchase of goods and services (option d) generally refers to consumption.