The correct answer is (c) Hydrogen bonds lengthen in the cold.
Ice is less dense than liquid water primarily because of the structure of ice and the hydrogen bonding between water molecules. In ice, water molecules are arranged in a specific lattice structure that causes them to be spaced further apart than in the liquid state. This is due to the hydrogen bonds forming at angles that create an open structure when water freezes, leading to lower density.
While (a) reflects the nature of hydrogen bonds in water, (b) is not directly relevant to the density comparison, and (c) accurately describes how hydrogen bonds behave as water transitions from liquid to solid. Therefore, the best explanation is found in option (c).