Resistivity unit.

**The reasoning** Resistivity (ρ) measures how strongly a material opposes current flow, *independent of its shape or size*. It's a property of the material itself. Start with Ohm's Law: R = V/I, so resistance R is measured in ohms (Ω). But resistance depends on the object's dimensions: **R = ρL/A** Where: - R = resistance (Ω) - L = length (m) - A = cross-sectional area (m²) Rearranging: **ρ = RA/L** Substituting units: ρ = (Ω × m²)/m = **Ω·m** That's why resistivity has units of ohm-meters! **Why the wrong options tempt you** **A) Ω** — This is just resistance, not resistivity. The trap: confusing the two related concepts. **C) V/m** — This looks scientific (it's actually electric field strength), so students might guess it relates to "voltage in materials." **D) A/m** — This is magnetic field intensity. The "per meter" makes it seem dimensional, but it's the wrong quantity entirely. **Quick takeaway** Resistivity isn't just resistance — it's resistance *adjusted for size*: multiply Ω by area, divide by length, and you get **Ω·m**.