Final answer:
The electricity for machinery of a food producer is a variable cost, as it varies with production output. Other listed costs, such as manager salaries for a clothing manufacturer or law firm, and electricity for office space, are generally considered to be fixed costs.
Step-by-step explanation:
To answer the question of which option is a variable cost, we should first understand what variable costs are. In business, variable costs are expenses that vary with production output. These costs rise as production increases and fall as production decreases. This can include costs such as labor, raw materials, and energy usage tied directly to manufacturing processes.
In the context provided, the electricity for machinery of a food producer would be a variable cost, because the amount of electricity used would increase if more food is produced and decrease if less is produced. Thus, it's directly tied to the production level. Other options like manager salaries for a clothing manufacturer or a law firm are typically fixed costs, as they do not usually change with the level of production or services provided. Similarly, electricity for office space tends to be a fixed cost because office activities don't typically fluctuate with production intensity.