Option b. warm water is less dense than cold water.
It is due to the the effect of heat on water molecules that's why warm water tend to rise above cold ones. The molecules of the water are excited by the energy. They begin moving faster, so when they bump into one another, they bounce further away. The increase space between the fast-moving molecules decreases the density.
Chemical, cause physical are changes affecting the form of a chemical substance, but not its chemical composition. This doesn't affect the substance but the composition.
