Which of the following is considered a component of operating expenses?

Salaries of sales staff are considered a component of operating expenses because these costs are directly tied to the day-to-day operations of a business. Operating expenses, often referred to as OPEX, encompass the costs required to run the business that are not directly linked to the production of goods or services. These can include selling, general, and administrative expenses, which are essential for maintaining the business's operational efficiency.

In this context, the salaries of sales staff contribute to the selling expenses, as these employees are critical in driving revenue through sales activities. Their salaries are recurring and necessary expenditures that the business incurs to generate sales.

Other components noted in the question do not qualify as operating expenses. For instance, while manufacturing costs can involve direct material and labor associated with production, they are typically classified under cost of goods sold (COGS) rather than operating expenses. Profits from investments reflect income generated from resources outside the primary activities of the business, which does not fall under operating expenses. Depreciation concerns the allocation of the cost of tangible assets over their useful lives, which relates to capital expenses rather than ongoing operational costs. Thus, the salaries of sales staff stand out as a clear example of an operating expense.