Final answer:
MgCl₂ is the substance that best conducts electricity in an aqueous solution among the given options.
Step-by-step explanation:
In an aqueous solution, the substance that best conducts electricity is a strong electrolyte. A strong electrolyte is a substance that dissociates completely into ions when dissolved in water, allowing for the movement of charged particles and conduction of electricity. Among the given options, the substance that is a strong electrolyte and conducts electricity the best is MgCl₂.