Final answer:
Paying interest expense and receiving interest revenue fall under operating activities, reflecting the use of financial capital sources like banks and bonds in running a firm's daily operations.
Step-by-step explanation:
Paying interest expense and receiving interest revenue are examples of operating activities. These transactions are part of a firm's everyday business operations, where it leverages different financial capital sources to fund its projects. While borrowing from banks or issuing bonds requires a firm to commit to scheduled interest payments, which are considered operating activities, selling stock transfers ownership and control but does not involve interest payments.