Elastic limit marks transition to plastic deformation

Steps:

• Recall elastic limit: maximum stress for reversible deformation.
• Beyond elastic limit, material undergoes permanent change.
• Analyze choices: identify statements matching elastic vs. plastic behavior.
• Select option accurately describing post-elastic limit effects.

Why A is correct:

• By definition, elastic limit is stress where plastic deformation begins; after it, deformation is irreversible and plastic.

Why the others are wrong:

• B: Extension measures length change, not a type like "plastic"; deformation is plastic.
• C: Proportionality (Hooke's law) holds only up to elastic limit, not after.
• D: Proportionality applies until elastic limit, but statement ignores post-limit plastic behavior.

Final answer: A