Work done equals force times extension for constant applied force

Steps:

• The applied force stretches the spring from unstretched length L0 to L1.
• The extension, or displacement of the end, is ΔL = L1 - L0.
• Work done by the constant force F over this displacement is W = F × ΔL.
• Substitute values to get W = F(L1 - L0).

Why B is correct:

• It follows the work formula W = F × displacement, where displacement is the extension L1 - L0 under constant force F.

Why the others are wrong:

• A: Assumes average force F/2 for variable loading like Hooke's law, but applied force is constant F.
• C: Multiplies by unstretched length L0, ignoring the actual displacement ΔL.
• D: Uses undefined L instead of specified L1 for the stretched length.

Final answer: B