Final answer:
Substance D, with a high melting point and the ability to conduct electricity in solution, is the most likely to be an ionic compound because these properties align with the known characteristics of ionic substances.
Step-by-step explanation:
To determine which substance is an ionic compound among A, B, C, and D, based on the given properties such as melting point, boiling point, and conductivity, we can apply our knowledge of the typical properties of ionic compounds. Ionic compounds have high melting and boiling points and conduct electricity when molten or dissolved in solution, but not when solid due to the fixed positions of the ions in the crystal lattice, which prevents them from moving freely to conduct electricity.
Substance A does not conduct electricity and has low melting and boiling points, suggesting it is not an ionic compound. Substance B also does not conduct electricity and has relatively low melting and boiling points. Substance C has a high melting point and conducts electricity as a solid, which is not typical of ionic compounds. Therefore, by process of elimination and comparing the properties to known ionic compounds (like sodium chloride, which melts at 801°C and boils at 1413°C), the best candidate is Substance D, which has a very high melting point (800°C) and conducts electricity in solution, consistent with the behavior of ionic compounds when dissolved or molten.