Answer:
A Monopolist has a downward sloping Demand curve which means that they will sell more goods if they charge lower prices.
Now Marginal Revenue is the change in the additional revenue that a company gets when it sells an extra unit. For this reason, the Marginal Revenue of a Monopoly is downward sloping as well because if the monopoly has to reduce their price to sell an additional unit, the additional unit will bring in less than the last unit.
The Marginal Revenue curve for a Monopoly is below the Demand Curve which gives them the opportunity to make an economic profit.
The point where the Marginal Revenue is equal to the Marginal Cost is the quantity where the Monopoly can maximise their profits as producing past this level will cost more than they are gaining per additional unit.