The correct answer to this question is D, Applies both to pure monopoly and pure competition. MC and MR are abbreviations related to the Profit Maximization Rule formula. The formula is MC = MR, in which MC stands for Marginal Cost.
MR stands for Marginal Revenue. When more units of goods are produced, the costs increases, which is the Marginal Cost. Marginal Revenue is the slop of the Total Revenue. It is when the rate of sales change, causing a change in the Total Revenue. MC = MR can relate to many things like how many times a TV commercial is running, as well as hours of operation.
This maximization rule simply states that if a firm (type doesn't matter) opts for maximizing its profits, it definitely has to choose such a level of output where Marginal Cost (MC) is equal to Marginal Revenue (MR). And Marginal Cost should be rising. We can also say, MR=MC.