Steepest slope indicates easiest stretch; flattest is hardest

Steps:

• Identify the graph as extension (y-axis) vs. load (x-axis) for threads A–D.
• Note that slope = extension/load, measuring stretchability (higher slope = easier to stretch).
• Compare slopes: smallest slope means least extension per load, so hardest to stretch.
• Select the line with the flattest slope over the load range.

Why D is correct:

• Thread D has the smallest slope, indicating lowest compliance (extension per unit load) per Hooke's law (F = kx, where larger k means stiffer material).

Why the others are wrong:

• A: Steeper slope shows greater extension, easier to stretch.
• B: Moderate slope indicates more stretch than D, less difficult.
• C: Highest slope means most extension, easiest to stretch.

Final answer: D