Which of the following is considered an operating expense?

Operating expenses are the costs that a company incurs through its normal business operations. They are necessary for the day-to-day functioning of the business, and they include costs related to running the company's core operations, such as rent, utilities, salaries, and repairs.

Repair costs for machinery fit this definition because they are necessary for maintaining equipment that is essential for operating the business. These expenses ensure that machinery remains functional, allowing the company to continue its production or service activities.

In contrast, the other options do not qualify as operating expenses. Proceeds from selling a piece of equipment represent an inflow of cash from an investing activity, rather than an expense. Capital investment in shares is also not an operating expense as it pertains to the acquisition of equity and is treated as an investment rather than a cost of running the business. Finally, income from interest on investments is categorized as non-operating income, as it does not correspond to the primary operations of the company, but rather results from external investments. Thus, repair costs for machinery is the only choice that aligns with the definition of an operating expense.