Modulus of elasticity (Young's modulus) for mild steel — choose the closest standard value. Give your answer in kN/mm^2 (note: 1 kN/mm^2 = 1 GPa).

Introduction:
Mild steel (low-carbon steel) is a standard structural material. This question tests recall of its elastic stiffness, commonly used in beam, column, shaft, and plate calculations in civil and mechanical engineering.

Given Data / Assumptions:

• Material: mild (low-carbon) steel.
• Small-strain, linear-elastic range.
• Units: kN/mm^2 where 1 kN/mm^2 = 1 GPa.

Concept / Approach:
Young's modulus E links normal stress and normal strain by Hooke's law: σ = Eε. For steels, E is nearly constant across grades and heat treatments within engineering accuracy, typically around 200 to 210 GPa (i.e., 200–210 kN/mm^2).

Step-by-Step Solution:

Verification / Alternative check:
Handbooks and codes often tabulate E_steel = 2.010^5 to 2.1*10^5 N/mm^2, which equals 200–210 kN/mm^2, confirming the chosen answer.

Why Other Options Are Wrong:

• 10, 25, 80, 100 kN/mm^2: Far lower than accepted values for steel; such stiffnesses are more typical of polymers (very low) or some non-ferrous alloys (still higher than listed low values).

Common Pitfalls:
Confusing strength (e.g., fy) with stiffness (E); mixing up units such as MPa, GPa, and N/mm^2. Remember: E for steels is relatively invariant compared to strength grades.

Final Answer: