Answer: B (concentrated aqueous and molten sodium chloride). The green gas is chlorine, Cl₂, produced at the anode (positive electrode).
Concentrated aqueous sodium chloride: the high concentration of chloride ions means chloride is discharged in preference to hydroxide, so green chlorine gas is released at the anode. Chlorine is formed.
Molten sodium chloride: only Na⁺ and Cl⁺ ions are present, so chloride is the only species available at the anode and chlorine gas is formed.
Dilute aqueous sodium chloride: with few chloride ions, hydroxide is discharged instead, giving oxygen at the anode, so no chlorine. Solid sodium chloride: the ions are locked in the lattice and cannot move, so no electrolysis occurs at all. Chlorine is therefore formed only in conditions 1 and 3, which is row B.
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