Stress is force per unit area

Steps:

• Stress is defined as force divided by cross-sectional area.
• Force has SI unit Newton (N), where 1 N = 1 kg m s^{-2}.
• Area has SI unit square meter (m²).
• Unit of stress is therefore N m^{-2} or kg m^{-1} s^{-2} (Pascal).

Why none are correct:

• The options do not match N/m²; question may have errors.
• A. kg^{-2}: Inverse mass squared; unrelated to force or area.
• B. kg m^{-2}: Mass per area (areal density); lacks time component for force.
• C. N m^{-1}: Force per length (e.g., surface tension); extra area dimension missing.
• D. N m: Energy or torque; multiplies by length instead of dividing.

Final answer: None; unit is N/m²