Strain as Fractional Extension Steps:

• Identify original length as L₀ and extended length as L₁.
• Calculate extension: ΔL = L₁ - L₀.
• Recall strain definition: ε = ΔL / original length.
• Substitute: ε = (L₁ - L₀) / L₀.

Why A is correct:

• Matches the definition of engineering strain: change in length divided by original length.

Why the others are wrong:

• B: Uses extended length L (likely L₁) in denominator, which defines true strain, not engineering strain.
• C: Adds lengths instead of subtracting, yielding positive nonsense value.
• D: Adds lengths with wrong denominator, unrelated to strain.

Final answer: A