Strain energy is the area under the force-extension graph for a spring.

Steps:

• The graph is linear, following Hooke's law, forming a right triangle from (0,0) to maximum extension and force.
• Strain energy U = (1/2) × base (extension) × height (force).
• From the graph, maximum extension = 0.2 m, maximum force = 6000 N.
• U = 0.5 × 0.2 × 6000 = 600 J.

Why C is correct:

• The formula for elastic strain energy in a linear spring is (1/2)Fe, where F is force and e is extension, yielding 600 J as the triangular area.

Why the others are wrong:

• A: 60 J ignores the 1/2 factor in the area formula.
• B: 120 J misapplies units or partial area calculation.
• D: 1200 J uses full rectangle area Fe instead of triangular (1/2)Fe.

Final answer: C