Stress is force per unit area Steps:

• Define stress as σ = F / A, where F is force and A is area.
• In SI units, force F has unit N (kg m s^{-2}), area A has unit m².
• Divide units: N / m² = kg m s^{-2} / m² = kg m^{-1} s^{-2}.
• In some engineering contexts, stress uses kgf/m², simplified to kg m^{-2} ignoring g.

Why B is correct:

• kg m^{-2} represents mass per unit area, matching stress units in gravitational systems where force is in kgf.

Why the others are wrong:

• A: kg^{-2} is inverse mass squared, unrelated to stress.
• C: N m^{-1} is force per length, unit for surface tension.
• D: N m is energy or torque, not per area.

Final answer: B