The correct answer is option 3 - a downward sloping straight line.

The average fixed cost (AFC) is calculated by dividing the total fixed cost (TFC) by the quantity produced. Since fixed costs do not change with the quantity produced, the AFC will decrease as the quantity produced increases. The graph of the AFC curve will be a straight line that slopes downward from left to right.

Option 1 - a rectangular hyperbola - is incorrect. A rectangular hyperbola is a curve where both the x and y values approach infinity. This does not represent the relationship between quantity produced and average fixed cost.

Option 2 - a downward sloping convex to the origin curve - is also incorrect. A downward sloping convex curve would imply that as the quantity produced increases, the AFC decreases at an increasing rate. This is not the case because the AFC decreases at a constant rate.

Option 4 - a U-shaped curve - is also incorrect. A U-shaped curve would imply that the AFC initially decreases, reaches a minimum point, and then starts increasing again. However, the AFC only decreases continuously as the quantity produced increases.

Therefore, option 3 - a downward sloping straight line - is the correct answer.