Plastic deformation involves permanent shape change after stress removal

Steps:

• Recall plastic deformation: material deforms irreversibly beyond elastic limit, retaining new shape.
• Evaluate each option for permanent vs. temporary deformation.
• Identify elastic recovery in options with rebound or sound.
• Confirm permanent bend in option showing lasting non-straightness.

Why C is correct:

• The plank bends under toolbox weight and stays deformed after removal, matching plastic deformation definition where stress exceeds yield point, causing irreversible strain per Hooke's law extension.

Why the others are wrong:

• A: Rubber ball compresses elastically and rebounds, showing temporary deformation.
• B: Clinking sound indicates elastic impact with no lasting change.
• D: Scale spring compresses elastically and returns to original shape upon unloading.

Final answer: C