Correct Answer :
B. Decreasing rate
In monopoly, TR curve increases at decreasing rate because monopolist or a producer under monopolistic competition can sell more of the commodity only by lowering its price and hence facing downward sloping AR(demand curve). So as sale increases, price or AR tends to reduce. If AR is reducing then MR must also reduce, hence it follows that TR under monopoly increases at a decreasing rate. So TR has no longer constant slope here.}