Final answer:
NaCl (sodium chloride) is the best conductor in the molten state among the options given, because it is an ionic compound whose ions are free to move and conduct electricity when melted.
Step-by-step explanation:
The compound that would be the best conductor in the molten state among the options given is NaCl (sodium chloride). This is because NaCl is an ionic compound, and ionic compounds consist of positively and negatively charged ions. When ionic compounds like NaCl are in the molten state, these ions are free to move, which allows the substance to conduct electricity. The other compounds listed are molecular in nature, they are made up of covalently bonded atoms which do not have free ions in molten state and therefore do not conduct electricity.