The correct answer is b) Vertical.

Explanation:
Price elasticity of demand measures the responsiveness of quantity demanded to a change in price. If the price elasticity of demand is zero, it means that the quantity demanded does not change at all when the price changes.

Shape of the curve:
The shape of the demand curve depends on the price elasticity of demand. If the price elasticity of demand is:

- Elastic (greater than 1): The demand curve is relatively flat or horizontal.
- Inelastic (less than 1): The demand curve is relatively steep or vertical.
- Unit elastic (equal to 1): The demand curve is a straight line with a 45-degree angle.

Therefore, if the price elasticity of demand is zero, it means that the demand is perfectly inelastic, and the demand curve is vertical.

Significance:
A perfectly inelastic demand means that consumers are willing to pay any price for the product, and they will continue to buy the same quantity regardless of the price. This situation is rare in real-world scenarios since most products have some substitutes, and consumers will switch to a substitute if the price of a product becomes too high.

Examples:
Some examples of products that have perfectly inelastic demand are:

- Life-saving drugs or medical treatments
- Unique works of art or collectibles
- Necessities such as water or electricity in the short run.

Conclusion:
In conclusion, if the price elasticity of demand is zero, the demand curve will be vertical, and the quantity demanded will not change with a change in price.

It means demand will not change whatever is the price so it will be vertical line parallel to y axis