The correct answer is A.
When a company operates in a market as the only producer (monopoly) has market power to fix a higher price and to not care too much about the quality of its products, as consumers do not have a different supplier from whom they could purchase the product instead.
Otherwise, if new firms appear in the market there is competition. If a different firm enters the market and offers the same product at a lower price, many consumers will switch and buy the product from the new competitor. The former monopolist is forced to lower the price too not to lose all its former customers.