The correct answer is A. Zero.
A vertical supply curve means that the quantity supplied is the same regardless of the price. This is because there is a fixed amount of the good available, and no matter how high the price goes, producers will not be able to supply any more. In this case, the elasticity of supply is zero, which means that the quantity supplied does not change at all in response to a change in price.
Option B, infinity, is incorrect. A supply curve with an elasticity of infinity would be horizontal, meaning that producers would be willing to supply any quantity of the good at any price. This is not realistic, as there are always some costs associated with production, and producers will not be willing to produce at a loss.
Option C, equal to one, is incorrect. A supply curve with an elasticity of one would be a straight line with a slope of one. This means that the quantity supplied would change by the same percentage as the price. This is not a realistic scenario, as the quantity supplied is usually more responsive to changes in price than this.
Option D, greater than zero but less than infinity, is incorrect. This is because a vertical supply curve implies that the elasticity of supply is zero.