Answer: D. Conduction via mobile electrons is a property of metals (and graphite, the only common non-metal exception). Ionic compounds conduct via mobile ions, not electrons. Covalent molecular substances do not conduct at all.
A — ammonia: a simple molecular covalent compound. No mobile electrons or ions. ✗
B — ammonium chloride: ionic. When molten or dissolved it conducts via \(\text{NH}_4^+\) and \(\text{Cl}^-\) ions, not electrons. ✗
C — cobalt(II) chloride: ionic. When liquid (molten), conducts via \(\text{Co}^{2+}\) and \(\text{Cl}^-\) ions, not electrons. ✗
D — cobalt: a transition metal. Even in the liquid state it retains a sea of delocalised valence electrons that carry charge. ✓
So cobalt is the only liquid here that conducts using mobile electrons.
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