Final answer:
KCl (potassium chloride) is the substance that best conducts electricity in an aqueous solution among the options because it dissociates into freely mobile potassium and chloride ions,
Step-by-step explanation:
The substance that best conducts electricity in an aqueous solution among CH₃OH, CO₂, KCl, and H₂O is KCl (potassium chloride). KCl is a salt made from potassium and chlorine, which dissociates into potassium ions (K+) and chloride ions (Cl-) when dissolved in water. These ions are freely mobile, charged species that make the solution a good conductor of electricity.
We can classify substances into electrolytes and nonelectrolytes based on their ability to conduct electricity in a solution. An electrolyte is a substance whose aqueous solution contains a significant concentration of ions which allows it to conduct electricity. On the other hand, a nonelectrolyte cannot conduct electricity well or at all because it does not produce ions in solution.
For instance, CH₃OH (“methanol”) and pure H₂O (water) are considered nonelectrolytes because when dissolved in water, they do not dissociate into ions that could carry an electrical current. CO₂ (carbon dioxide) also does not produce ions in water and is a nonelectrolyte. However, KCl is a strong electrolyte, which means that it completely dissociates into ions in water, contributing to high electrical conductivity. Therefore, the answer to the student's question is KCl (C. KCl).