b. It is the same as the market demand curve.

A monopoly firm is a single seller of the product in the market and it has full control over the market supply. The difference between firm and industry disappears in the case of a monopoly market. Since a monopoly is a single seller in the market so the demand curve of the monopoly is the same as the market demand curve. A monopoly firm is like the industry as a single seller constitutes the entire market for the product, which has no close substitute. So, a monopolist has full freedom and power to fix the price for the product. However, demand for the product is not in the control of a monopoly firm. In order to increase the output to be sold, the monopolist will have to reduce the price. Therefore, a monopoly firm faces a downward-sloping demand curve.

The demand curve is less elastic in the monopoly market as compared to the monopolistically competitive market due to the absence of close substitutes.