Dear student,
The strength of conducting Electricity by an 'Electrolyte' depends upon the following factors :
1. Molarity ( Concentration )
2. Degree of dissociation
3. Atmospheric ( Parameters )
As the given electrolytes have same concentration and atmospheric parameters, therefore the one with maximum degree of dissociation would be the best conductor of electricity.
Hence, (D) NaCl, which is the most ionic salt among all, would dissociate the most, so would be the best conductor of electricity.
Regards,