The correct answer is A. Vertical.
A perfectly inelastic supply curve is a vertical line. This means that the quantity supplied will not change regardless of the change in price. This is because the supplier has no incentive to change the quantity supplied, even if the price changes.
A perfectly elastic supply curve is a horizontal line. This means that the quantity supplied will change infinitely in response to a change in price. This is because the supplier is willing to supply any quantity at the market price.
An upward-sloping supply curve is a curve that slopes upwards from left to right. This means that the quantity supplied will increase as the price increases. This is because the supplier is willing to supply more at a higher price.
A downward-sloping supply curve is a curve that slopes downwards from left to right. This means that the quantity supplied will decrease as the price increases. This is because the supplier is willing to supply less at a higher price.