Limit of proportionality on a spring's extension-load graph

Steps:

• Recall Hooke's law: force is proportional to extension, producing a straight line through origin.
• Identify the straight-line portion of the graph, where load vs. extension is linear.
• Locate the point where the line begins to curve, marking deviation from proportionality.
• Select the option at that deviation point.

Why B is correct:

• Point B is where the graph stops being a straight line, ending the proportional region per Hooke's law (F = kx).

Why the others are wrong:

• A: Lies on the initial straight line, within proportionality.
• C: Occurs after deviation, in non-proportional elastic region.
• D: Marks yield point or plastic deformation, beyond proportionality.

Final answer: B