The correct option is D Mobility of K+ and Cl− ions are almost same
Salt bridge is a U–shaped inverted tube that contains a gel (paste of agar-agar powder) permeated with an inert electrolyte (eg: KCl)
Salt bridge connects the cathode and anode electrode internally.
Criteria for electrolyte in a salt bridge:
1. The electrolytes should consist of cations and anions having same mobility.
2. It should not react with the ions in the solution.
3. It should not oxidised/reduced at the electrodes
KCl has all these 3 criteria and is generally preferred for salt bridge.