What is the unit of specific resistance?

Correct — C. Specific resistance is another name for resistivity, the constant in R = ρL/A.

Rearrange for ρ: ρ = RA/L
Units: ohm × m² ÷ m
= ohm metre (Ω·m)

The extra metre is the whole point. Resistance belongs to a particular piece of wire and changes with its length and thickness; resistivity belongs to the material and does not.

Why the others are wrong

• (a)
  Coulomb — The coulomb is the unit of electric charge, one ampere flowing for one second. It measures how much charge passed, not how strongly a substance opposes current.
• (b)
  Ampere — The ampere measures current, the rate of flow of charge, and is an SI base unit. Resistivity is a property of the material, not of the current in it.
• (d)
  Farad — The farad is the unit of capacitance, one coulomb stored per volt. It describes a capacitor, not a material's opposition to current.

Resistance belongs to an object; resistivity belongs to a substance.

Take two copper wires, one twice as long as the other. The longer wire has twice the resistance, but both have exactly the same resistivity.

That is why R = ρL/A carries the geometry — length L and cross-sectional area A — while ρ carries only the material.

The reciprocal of resistivity is conductivity, measured in siemens per metre. Resistivity of a metal rises as it gets hotter; for a semiconductor it falls.

Key facts

• Resistivity, also called specific resistance, has the SI unit ohm metre, written as ohm·m.
• R = ρL/A, so resistance depends on the sample's length and cross-section while resistivity does not.
• Conductivity is the reciprocal of resistivity and is measured in siemens per metre.
• Metals become more resistive when heated, while semiconductors become less resistive.

Common traps

• Answering 'ohm' from habit — the stem asks for specific resistance, which carries an extra metre.
• Expecting resistivity to change when you cut the wire shorter.

No directly related past PYQ was found.