What must break in order for water to change from solid to liquid to gas?
a. covalent bonds between water molecules
b. ionic bonds within water molecules
c. hydrogen bonds within water molecules
d. hydrogen bonds between water molecules
e. covalent bonds within water molecules

When changing states, only the intermolecular forces are considered, which, in water's case are hydrogen bonds. In order to go from solid to liquid to gas, the hydrogen bonds between the water molecules must be weakened to a certain degree that allows the molecules to spread further apart in liquids, and must be broken altogether to separate the molecules in order to turn it into gas.
Therefore, the answer is d.

The reason it is not the covalent bonds within molecules (despite the fact that it will turn water into a gas, it’s not water vapour, but hydrogen and oxygen gas) is that when bonds within a molecule are broken, this is called decomposition and it is a CHEMICAL change. Changing states is only a PHYSICAL change.
It is not ionic because water has covalent bonds.
Hydrogen bonds can only occur BETWEEN molecules (intermolecular forces).
Covalent bonds can only occur WITHIN molecules (intramolecular).

C. which metal is less dense

Ido not understand.