The answer is D.

The power a firm has in the market is described when the demand curve is (d) downward sloping. When the curve is sloping downwards, a firm can change the price of the commodity by changing quantity thus manipulating demand and supply. When the curve is a vertical line, it implies that the firm can only produce at a fixed quantity thus lacks the power to control prices. When the curve is horizontal then demand for the particular product is perfectly elastic at a fixed price thus limiting the power of the firm.