The correct option is D Brine solution flow in opposite direction of mercury
In Castner-Kellner cell, mercury is used as cathode and it flow along the bottom of the cell. The brine solution flow in same direction of mercury and they react with mercury to form mercury amalgam.
Mercury amalgam on reacting with water gives NaOH, Hg and H2.
2NaHg+2H2O→2NaOH+2Hg+H2
Hg is re-circulated to the cell for cyclic process. Cl2 gas is liberated from the anode. Hence, H2 and Cl2 were the usefull by-products of the reaction.
Hence, the wrong statement is (d).