Assertion :Kohlrausch law helps to find the molar conductivity of weak electrolyte at infinite dilution. Reason: Molar conductivity of a weak electrolyte at infinite dilution cannot be determined experimentally.

The correct option is A Both Assertion and Reason are correct and Reason is the correct explanation for Assertion

The molar conductivity  of weak electrolytes at infinite dilution cannot be 
determined experimentally, firstly because the  conductance of such a solution is 
low and secondly because dissociation of  such an electrolytes is not  completed 
even at high dilutions. 
The molar conductivity of such an electrolyte at infinite dilution can be 
calculated using Kohlrausch’s law.
Kohlrausch's law  states that the equivalent conductivity of an 
electrolyte at infinite dilution is equal to the sum of the 
conductances of the anions and cations. If a salt 
is dissolved in water, the conductivity of the solution 
is the sum of the conductances of the anions and cations.
Hence,Both Assertion and reason are correct.