Elastic limit as transition to plastic deformation

Steps:

• Recall stress-strain graph: linear elastic region up to elastic limit, then plastic.
• Elastic limit strain is maximum for elastic behavior (Hooke's law) and start of plastic.
• Evaluate choices: identify which defines boundary where plastic deformation begins.
• Select phrase matching minimum strain for plastic onset.

Why D is correct:

• Elastic limit is defined as the strain where plastic deformation starts, per material science: minimum strain above which deformation is permanent and non-Hookean.

Why the others are wrong:

• A: Describes maximum elastic strain but phrasing "below which Hooke's law is obeyed" implies strict inequality, not the limit point.
• B: Incorrectly states plastic deformation below limit; it's elastic below.
• C: Reverses logic—Hooke's law holds below, not above, elastic limit.

Final answer: D