Why is water less dense when it is in a frozen state.

Your answer is, close to the freezing point, the water molecules start to arrange locally into ice-like structures. This creates some "openness" in the liquid water, which tends to decrease its density.

Hope this helps!

c- cold periods in earth's history during which glaciers covered much of the surface.

explanation:

i'm assuming that the correct answer is indeed the first one/ "a"..