The correct answer is:
C. the density of the substance decreases.
In general, when a substance changes from a solid to a liquid (melting), the structure of the molecules typically becomes less ordered and may occupy a larger volume in the liquid state compared to the solid state, leading to a decrease in density. However, there are exceptions (like ice, which is less dense than liquid water), but for most substances, the density decreases when transitioning from solid to liquid.