Question

though you add heat, the temperature of boiling water remains constant because ...
it takes energy to break covalent bonds.
it takes energy to circulate water.
it takes energy to break hydrogen bonds.
water has a constant boiling temperature.
none of the above. the temperature rises during boiling.

Explanation:

When water boils, the added heat energy is used to break the hydrogen - bonds between water molecules, not to increase the temperature until all water is converted to steam. Covalent bonds within water molecules don't break during boiling. Circulating water isn't the reason for constant temperature during boiling. And the temperature doesn't rise during the boiling process at a constant pressure once the boiling starts.

Answer:

it takes energy to break hydrogen bonds.