What causes the waxy substance in a lava lamp to rise?

The correct answer is that the waxy substance is less dense than the surrounding liquid.

In a lava lamp, the heating element at the bottom warms the wax, causing it to expand. As the wax heats up, it undergoes a decrease in density relative to the surrounding liquid (typically water-based). When the wax becomes less dense, it rises through the denser liquid above it, which is a fundamental principle of convection. As the wax rises, it cools down, becomes denser again, and then sinks back down where it is reheated. This continuous cycle creates the characteristic movement seen in a lava lamp.

The other options describe properties that do not facilitate the rising of the wax. If the wax were cold or more dense than the surrounding liquid, it would either remain at the bottom or sink further, rather than rise.