When a substance changes from a solid to a liquid, the general trend is that the density of the substance may change, often decreasing due to the increase in volume that occurs in many cases. Therefore, the most accurate answer in most cases is:
D. the density of the substance decreases
However, it's worth noting that there are exceptions (like water, which is less dense in its solid form as ice than in its liquid form). But in general, solid to liquid transitions often result in a decrease in density.