The correct answer is A, chlorine.
- The question describes the electrolysis of concentrated aqueous sodium chloride, which is also known as brine.
- The main ions present are Na+, Cl−, H+, and OH−.
- At the positive electrode (anode), anions are attracted. The competing anions are Cl− and OH−.
- In concentrated sodium chloride solution, there is a very high concentration of Cl− ions. The high concentration makes it more likely for chloride ions to be discharged at the anode than hydroxide ions.
- The product formed is chlorine gas.
- The other products are hydrogen gas at the negative electrode (cathode) and sodium hydroxide in the solution.