Kohlrausch's Law helps to find:

The correct answer is Option (2) → The molar conductivity of a weak electrolyte at infinite dilution.

Kohlrausch law of independent migration of ions: the limiting molar conductivity of an electrolyte is the sum of the individual contributions of its anion and cation:

    \( \Lambda^\circ_m = \nu_+\lambda^\circ_+ + \nu_-\lambda^\circ_- \)

For a strong electrolyte, Λ_m plotted against √c is a straight line, and Λ°_m is read by extending the line to zero concentration. For a weak electrolyte, Λ_m rises steeply at low concentration, so this extrapolation is not possible. Its Λ°_m is then found from the Λ° values of strong electrolytes by Kohlrausch law, for example:

    \( \Lambda^\circ_m(\ce{CH3COOH}) = \Lambda^\circ_m(\ce{CH3COONa}) + \Lambda^\circ_m(\ce{HCl}) - \Lambda^\circ_m(\ce{NaCl}) \)

Why the other options are incorrect

• Option (1) The molar conductivity of a concentrated weak electrolyte. — Incorrect. The law deals with ions at infinite dilution, where they move independently. It gives no value for a concentrated solution.

• Option (3) The molar conductivity of a concentrated strong electrolyte. — Incorrect. In a concentrated solution, attractions between ions lower the conductivity, so the ions do not move independently and the law does not apply.

• Option (4) The molar conductivity of a strong electrolyte at infinite dilution. — Incorrect. The law holds for strong electrolytes too, but their Λ°_m is found directly by extrapolation. The law is needed for weak electrolytes, where extrapolation fails.

NCERT (2026-27 edition):
Weak Electrolytes – “Weak electrolytes like acetic acid have lower degree of dissociation at higher concentrations and hence for such electrolytes, the change in Λ_m with dilution is due to increase in the degree of dissociation and consequently the number of ions in total volume of solution that contains 1 mol of electrolyte. In such cases Λ_m increases steeply (Fig. 2.6) on dilution, especially near lower concentrations. Therefore, Λ°_m cannot be obtained by extrapolation of Λ_m to zero concentration. At infinite dilution (i.e., concentration c → zero) electrolyte dissociates completely (α = 1), but at such low concentration the conductivity of the solution is so low that it cannot be measured accurately. Therefore, Λ°_m for weak electrolytes is obtained by using Kohlrausch law of independent migration of ions (Example 2.8).”