The correct answer is D. A supply curve will have a price elasticity equal to 1 only when it is horizontal.
A horizontal supply curve is a straight line that is parallel to the horizontal axis. This means that the quantity supplied is perfectly elastic, or that the quantity supplied will change by an infinite amount in response to a change in price.
A straight line with a positive intercept is a supply curve that slopes upward from left to right. This means that the quantity supplied increases as the price increases.
A straight line with a negative intercept is a supply curve that slopes downward from left to right. This means that the quantity supplied decreases as the price increases.
A straight line passing through the origin is a supply curve that has a slope of 0. This means that the quantity supplied does not change in response to a change in price.
In conclusion, a supply curve will have a price elasticity equal to 1 only when it is horizontal.