**Conductivity** $\kappa$ of a solution refers to:
- Conductance of 1 mole of electrolyte in solution
- Conductance of a 1 m³ cube
- Resistance of any volume of solution
- Conductance of a unit-length, unit-area cube of solution
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Correct answer: D. Conductance of a unit-length, unit-area cube of solution
Conductivity is the conductance of a conductor of **unit length and unit cross-sectional area** (a unit cube).
**Units:** S m⁻¹ (SI) or Ω⁻¹ cm⁻¹.