Marginal Cost is the variation (increase or decrease) of the costs of a production run when is increased by one additional unit of any given item, and it is calculated when the <em>breakeven point</em> has already been already reached.
Its usage is crucial for companies because it allows them to set a parameter that defines its optimum production quantity (also known as the "sweet spot") in which, if they operate within, it can maximize its profits.
Therefore, the answer is B: The price of producing one additional unit of a good.
