Resistivity unit from resistance formula

Steps:

• Ohm's law gives resistance R in ohms (Ω).
• For a material, R = ρ L / A, where L is length in meters (m), A is cross-sectional area in m².
• Solve for resistivity: ρ = R A / L.
• Substitute units: Ω × m² / m = Ωm.

Why D is correct:

• From ρ = R A / L, the SI unit is ohm-meter (Ωm), measuring resistance per unit length and area.

Why the others are wrong:

• A: Ωm² is the unit for R × A, not divided by length.
• B: Ωm duplicates D but is not standard notation; resistivity requires the full Ωm form.
• C: Ω is the unit for resistance R alone, without geometric factors.

Final answer: D