Young's Modulus Definition Steps:

• Recall that Young's modulus measures a material's stiffness under tension.
• Stress σ is force per unit area, and strain ε is fractional change in length.
• Young's modulus Y is defined as the ratio of stress to strain for elastic deformation.
• Thus, Y = σ / ε, matching option A.

Why A is correct:

• It directly follows the formula Y = stress / strain = σ / ε.

Why the others are wrong:

• B inverts the ratio, giving strain over stress, which is compliance, not modulus.
• C is just stress, ignoring strain and lacking the ratio needed for modulus.
• D involves strain and area, unrelated to the stress-strain ratio for Young's modulus.

Final answer: A