For a horizontal demand curve
a. the slope is equal to 0, and the price elasticity of demand is undefined.
b. the slope is undefined, and the price elasticity of demand is equal to 0.
c. both the slope and price elasticity of demand are undefined.
d. both the slope and price elasticity of demand are equal to 0.

d. both the slope and price elasticity of demand are equal to 0.

Step-by-step explanation:

In order to graph the demand curve, the quantity demanded is plotted along x-axis and the price is plotted along y-axis. An image attached below shows the horizontal demand curve.

Horizontal demand curve, as its name indicates, is a horizontal line which is parallel to x-axis. Since, the slope of any line parallel to x-axis is 0, we can conclude that the slope of Horizontal demand curve is 0.

A horizontal demand curve can be observed for a perfectly competitive market. Since, its a perfect competition, the price of a product by all competitors will be the same. In this case, if a firm decides to increase the price, he will loose his market share as no customer will buy the product at increased price. They will rather go with the other competitor who is offering a similar product at lower price.

On the other hand, if a competitor decides to lower his price in such case, he will experience loss. Therefore, the competitors do not have the option to change the price. Therefore, we can say the price elasticity of demand in this case is 0.

So, option D describes the horizontal demand curve correctly.