Answer: D. The phrase 'same element oxidised at one electrode and reduced at the other' identifies a metal-refining type of electrolysis.
Experiment 1 (dilute NaCl, graphite): oxygen is released at the anode (from water), hydrogen at the cathode (from water) — two different elements.
Experiment 2 (concentrated NaCl, graphite): chlorine at the anode, hydrogen at the cathode — different elements.
Experiment 3 (\(\text{CuSO}_4\), copper electrodes): at the anode, copper is oxidised: \(\text{Cu} \rightarrow \text{Cu}^{2+} + 2\text{e}^{-}\). At the cathode, copper is reduced: \(\text{Cu}^{2+} + 2\text{e}^{-} \rightarrow \text{Cu}\). Same element — copper — at both electrodes. This is the electrorefining set-up.
Only experiment 3 fits → answer D.
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