Unit Elastic Supply Curve

Steps:

• Recall price elasticity of supply (PES) measures responsiveness of quantity supplied to price changes; PES=1 indicates unit elasticity.
• Identify supply curve shapes: vertical (PES=0), horizontal (PES=∞), linear through origin (PES=1 constant), other slopes (PES varies).
• Examine each diagram for constant PES=1, which occurs only on a ray from the origin where slope ensures %ΔQ = %ΔP.
• Select the curve matching this property.

Why D is correct:

• Diagram D shows a linear supply curve passing through the origin, where PES = (dQ/dP)(P/Q) = 1 at every point by definition.

Why the others are wrong:

• A: Vertical curve means PES=0 (no quantity response to price).
• B: Horizontal curve means PES=∞ (infinite quantity response).
• C: Upward-sloping but not through origin means PES varies along the curve.

Final answer: D